Professor James Jackson

Professor James Jackson

Head of School

School of Mathematical and Physical Sciences

Career Summary

Biography

My research has focussed on the formation of high-mass stars, primarily by using observations from radio and infrared telescopes.  I have led three large surveys of star-forming molecular gas clouds: the Galactic Ring Survey (13-carbon monoxide), the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT 90; various molecules such as HCN, HNC, HCO+, and N2H+), and the Radio Astronomy Mid-Plane Survey (RAMPS).  My collaborators and I helped to establish the important role of Infrared Dark Clouds as the sites of the earliest stages of high-mass star formation.


Qualifications

  • PhD (Physics), Massachusetts Institute of Technology - USA

Keywords

  • Astronomy
  • Astrophysics
  • Galactic astronomy
  • star formation

Fields of Research

Code Description Percentage
020104 Galactic Astronomy 100

Professional Experience

UON Appointment

Title Organisation / Department
Professor University of Newcastle
School of Mathematical and Physical Sciences
Australia

Professional appointment

Dates Title Organisation / Department
1/07/2011 - 31/08/2014 Associate Dean for Research and Outreach Boston University MA
College of Arts and Sciences
United States
1/01/2006 - 31/12/2010 Chair Boston University MA
Department of Astronomy
United States
1/01/2001 - 31/12/2005 Director Boston University MA
Institute for Astrophysical Research
United States
1/01/2001 - 31/07/2016 Professor of Astronomy Boston University MA
United States
1/01/1996 - 31/12/2000 Associate Professor of Astronomy Boston University MA
United States
1/01/1996 - 31/12/1999 Assistant Director Center for Astrophysical Research
Antarctica
1/01/1991 - 31/12/1995 Assistant Professor of Astronomy Boston University MA
United States
1/01/1988 - 1/01/1990 Physicist Max Planck Institute for Physics and Astrophysics
Germany
1/01/1986 - 31/12/1987 Post-doctoral Radio Astronomer University of California, Berkeley
United States
Edit

Publications

For publications that are currently unpublished or in-press, details are shown in italics.


Journal article (143 outputs)

Year Citation Altmetrics Link
2017 Sanhueza P, Jackson JM, Zhang Q, Guzman AE, Lu X, Stephens IW, et al., 'A Massive Prestellar Clump Hosting No High-mass Cores', ASTROPHYSICAL JOURNAL, 841 (2017) [C1]
DOI 10.3847/1538-4357/aa6ff8
2017 Whitaker JS, Jackson JM, Rathborne JM, Foster JB, Contreras Y, Sanhueza P, et al., 'MALT90 Kinematic Distances to Dense Molecular Clumps', ASTRONOMICAL JOURNAL, 154 (2017)
DOI 10.3847/1538-3881/aa86ad
2016 Federrath C, Rathborne JM, Longmore SN, Kruijssen JMD, Bally J, Contreras Y, et al., 'THE LINK between TURBULENCE, MAGNETIC FIELDS, FILAMENTS, and STAR FORMATION in the CENTRAL MOLECULAR ZONE CLOUD G0.253+0.016', Astrophysical Journal, 832 (2016)

© 2016. The American Astronomical Society. All rights reserved. Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However,... [more]

© 2016. The American Astronomical Society. All rights reserved. Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic center may differ substantially compared to spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field, and filamentary structure. Using column density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width and the sonic scale of the turbulence, and find W fil ¿ ¿ sonic . A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra. The gradient is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity probability distribution function (PDF). After subtracting the gradient to isolate the turbulent motions, we find a nearly Gaussian velocity PDF typical for turbulence. We measure the total and turbulent velocity dispersion, 8.8 ± 0.2 km s -1 and 3.9 ± 0.1 km s -1 , respectively. Using magnetohydrodynamical turbulence simulations, we find that G0.253+0.016's turbulent magnetic B turb = 130 ± 50 µG is only ¿ 1/10 only of the ordered field component. Combining these measurements, we reconstruct the dominant turbulence driving mode in G0.253+0.016 and find a driving parameter of b = 0.22 ± 0.12, indicating solenoidal (divergence-free) driving. We compare this to spiral-arm clouds, which typically have a significant compressive (curl-free) driving component (b > 0.4). Motivated by previous reports of strong shearing motions in the CMZ, we speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this reduces the star-formation rate by a factor of 6.9 compared to typical nearby clouds.

DOI 10.3847/0004-637X/832/2/143
Citations Scopus - 3
2016 Stephens IW, Jackson JM, Whitaker JS, Contreras Y, Guzmán AE, Sanhueza P, et al., 'LINKING DENSE GAS from the MILKY WAY to EXTERNAL GALAXIES', Astrophysical Journal, 824 (2016)

© 2016. The American Astronomical Society. All rights reserved. In a survey of 65 galaxies, Gao & Solomon found a tight linear relation between the infrared luminosity (L I... [more]

© 2016. The American Astronomical Society. All rights reserved. In a survey of 65 galaxies, Gao & Solomon found a tight linear relation between the infrared luminosity (L IR , a proxy for the star formation rate) and the HCN(1-0) luminosity (L HCN ). Wu et al. found that this relation extends from these galaxies to the much less luminous Galactic molecular high-mass star-forming clumps (~1 pc scales), and posited that there exists a characteristic ratio L IR /L HCN for high-mass star-forming clumps. The Gao-Solomon relation for galaxies could then be explained as a summation of large numbers of high-mass star-forming clumps, resulting in the same L IR /L HCN ratio for galaxies. We test this explanation and other possible origins of the Gao-Solomon relation using high-density tracers (including HCN(1-0), N 2 H + (1-0), HCO + (1-0), HNC(1-0), HC 3 N(10-9), and C 2 H(1-0)) for ~300 Galactic clumps from the Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey. The MALT90 data show that the Gao-Solomon relation in galaxies cannot be satisfactorily explained by the blending of large numbers of high-mass clumps in the telescope beam. Not only do the clumps have a large scatter in the L IR / L HCN ratio, but also far too many high-mass clumps are required to account for the Galactic IR and HCN luminosities. We suggest that the scatter in the L IR / L HCN ratio converges to the scatter of the Gao-Solomon relation at some size-scale ¿1 kpc. We suggest that the Gao-Solomon relation could instead result from of a universal large-scale star formation efficiency, initial mass function, core mass function, and clump mass function.

DOI 10.3847/0004-637X/824/1/29
Citations Scopus - 2
2016 Rathborne JM, Whitaker JS, Jackson JM, Foster JB, Contreras Y, Stephens IW, et al., 'Molecular Line Emission Towards High-Mass Clumps: The MALT90 Catalogue', Publications of the Astronomical Society of Australia, (2016)

Copyright © Astronomical Society of Australia 2016 The Millimetre Astronomy Legacy Team 90 GHz survey aims to characterise the physical and chemical evolution of high-mass clumps... [more]

Copyright © Astronomical Society of Australia 2016 The Millimetre Astronomy Legacy Team 90 GHz survey aims to characterise the physical and chemical evolution of high-mass clumps. Recently completed, it mapped 90 GHz line emission towards 3 246 high-mass clumps identified from the ATLASGAL 870 µm Galactic plane survey. By utilising the broad frequency coverage of the Mopra telescope¿s spectrometer, maps in 16 different emission lines were simultaneously obtained. Here, we describe the first catalogue of the detected line emission, generated by Gaussian profile fitting to spectra extracted towards each clumps¿ 870 µm dust continuum peak. Synthetic spectra show that the catalogue has a completeness of > 95%, a probability of a false-positive detection of < 0.3%, and a relative uncertainty in the measured quantities of < 20% over the range of detection criteria. The detection rates are highest for the (1¿0) transitions of HCO + , HNC, N 2 H + , and HCN (~77¿89%). Almost all clumps (~95%) are detected in at least one of the molecular transitions, just over half of the clumps (~53%) are detected in four or more of the transitions, while only one clump is detected in 13 transitions. We find several striking trends in the ensemble of properties for the different molecular transitions when plotted as a function of the clumps¿ evolutionary state as estimated from Spitzer mid-IR images, including (1) HNC is relatively brighter in colder, less evolved clumps than those that show active star formation, (2) N 2 H + is relatively brighter in the earlier stages, (3) that the observed optical depth decreases as the clumps evolve, and (4) the optically thickest HCO + emission shows a ¿blue-red asymmetry¿ indicating overall collapse that monotonically decreases as the clumps evolve. This catalogue represents the largest compiled database of line emission towards high-mass clumps and is a valuable data set for detailed studies of these objects.

DOI 10.1017/pasa.2016.23
2015 Jordan CH, Walsh AJ, Lowe V, Voronkov MA, Ellingsen SP, Breen SL, et al., 'MALT-45: A 7 mm survey of the southern Galaxy - I. Techniques and spectral line data', Monthly Notices of the Royal Astronomical Society, 448 2344-2361 (2015)

© 2015 The Authors. We present the first results from the MALT-45 (Millimetre Astronomer&apos;s Legacy Team- 45 GHz) Galactic Plane survey. We have observed 5 square degrees (l=3... [more]

© 2015 The Authors. We present the first results from the MALT-45 (Millimetre Astronomer's Legacy Team- 45 GHz) Galactic Plane survey. We have observed 5 square degrees (l=330°-335°, b=±0.°5) for spectral lines in the 7mmband (42-44 and 48-49 GHz), including CS (1-0), class I CH 3 OH masers in the 7(0,7)-6(1,6) A + transition and SiO (1-0) v = 0, 1, 2, 3. MALT-45 is the first unbiased, large-scale, sensitive spectral line survey in this frequency range. In this paper, we present data from the survey as well as a few intriguing results; rigorous analyses of these science cases are reserved for future publications. Across the survey region, we detected 77 class I CH 3 OH masers, of which 58 are new detections, along with many sites of thermal and maser SiO emission and thermal CS. We found that 35 class I CH 3 OH masers were associated with the published locations of class II CH 3 OH, H 2 O and OH masers but 42 have no known masers within 60 arcsec. We compared the MALT-45 CS with NH 3 (1,1) to reveal regions of CS depletion and high opacity, as well as evolved star-forming regions with a high ratio of CS to NH3. All SiO masers are new detections, and appear to be associated with evolved stars from the Spitzer Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE). Generally, within SiO regions of multiple vibrational modes, the intensity decreases as v = 1, 2, 3, but there are a few exceptions where v = 2 is stronger than v = 1.

DOI 10.1093/mnras/stv178
Citations Scopus - 8
2015 Deharveng L, Zavagno A, Samal MR, Anderson LD, Leleu G, Brevot D, et al., 'Bipolar H II regions - Morphology and star formation in their vicinity *: I. G319.88 + 00.79 and G010.32 - 00.15', Astronomy and Astrophysics, 582 (2015)

© ESO, 2015. Aims. Our goal is to identify bipolar H ii regions and to understand their morphology, their evolution, and the role they play in the formation of new generations of... [more]

© ESO, 2015. Aims. Our goal is to identify bipolar H ii regions and to understand their morphology, their evolution, and the role they play in the formation of new generations of stars. Methods. We use the Spitzer-GLIMPSE, -MIPSGAL, and Herschel-Hi-GAL surveys to identify bipolar H ii regions, looking for (ionized) lobes extending perpendicular to dense filamentary structures. We search for their exciting star(s) and estimate their distances using near-IR data from the 2MASS or UKIDSS surveys. Dense molecular clumps are detected using Herschel-SPIRE data, and we estimate their temperature, column density, mass, and density. MALT90 observations allow us to ascertain their association with the central H ii region (association based on similar velocities). We identify Class 0/I young stellar objects (YSOs) using their Spitzer and Herschel-PACS emissions. These methods will be applied to the entire sample of candidate bipolar H ii regions to be presented in a forthcoming paper. Results. This paper focuses on two bipolar H ii regions, one that is especially interesting in terms of its morphology, G319.88+00.79, and one in terms of its star formation, G010.32-00.15. Their exciting clusters are identified and their photometric distances estimated to be 2.6 kpc and 1.75 kpc, respectively; thus G010.32-00.15 (known as W31 north) lies much closer than previously assumed. We suggest that these regions formed in dense and flat structures that contain filaments. They have a central ionized region and ionized lobes extending perpendicular to the parental cloud. The remains of the parental cloud appear as dense (more than 10 < sup > 4 < /sup > cm < sup > -3 < /sup > ) and cold (14-17 K) condensations. The dust in the photodissociation regions (in regions adjacent to the ionized gas) is warm (19-25 K). Dense massive clumps are present around the central ionized region. G010.32-00.14 is especially remarkable because five clumps of several hundred solar masses surround the central H ii region; their peak column density is a few 10 < sup > 23 < /sup > cm < sup > -2 < /sup > , and the mean density in their central regions reaches several 10 < sup > 5 < /sup > cm < sup > -3 < /sup > . Four of them contain at least one massive YSO (including an ultracompact H ii region and a high-luminosity Class I YSO); these clumps also contain extended green objects (EGOs) and Class II methanol masers. This morphology suggests that the formation of a second generation of massive stars has been triggered by the central bipolar H ii region. It occurs in the compressed material of the parental cloud.

DOI 10.1051/0004-6361/201423835
Citations Scopus - 13
2015 Guzmán AE, Sanhueza P, Contreras Y, Smith HA, Jackson JM, Hoq S, Rathborne JM, 'FAR-INFRARED DUST TEMPERATURES and COLUMN DENSITIES of the MALT90 MOLECULAR CLUMP SAMPLE', Astrophysical Journal, 815 (2015)

© 2015. The American Astronomical Society. All rights reserved. We present dust column densities and dust temperatures for ~3000 young, high-mass molecular clumps from the Millim... [more]

© 2015. The American Astronomical Society. All rights reserved. We present dust column densities and dust temperatures for ~3000 young, high-mass molecular clumps from the Millimeter Astronomy Legacy Team 90 GHz survey, derived from adjusting single-temperature dust emission models to the far-infrared intensity maps measured between 160 and 870 µm from the Herschel/Herschel Infrared Galactic Plane Survey (Hi-Gal) and APEX/APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) surveys. We discuss the methodology employed in analyzing the data, calculating physical parameters, and estimating their uncertainties. The population average dust temperature of the clumps are 16.8 ± 0.2 K for the clumps that do not exhibit mid-infrared signatures of star formation (quiescent clumps), 18.6 ± 0.2 K for the clumps that display mid-infrared signatures of ongoing star formation but have not yet developed an H ii region (protostellar clumps), and 23.7 ± 0.2 and 28.1 ± 0.3 K for clumps associated with H ii and photo-dissociation regions, respectively. These four groups exhibit large overlaps in their temperature distributions, with dispersions ranging between 4 and 6 K. The median of the peak column densities of the protostellar clump population is 0.20 ± 0.02 g cm -2 , which is about 50% higher compared to the median of the peak column densities associated with clumps in the other evolutionary stages. We compare the dust temperatures and column densities measured toward the center of the clumps with the mean values of each clump. We find that in the quiescent clumps, the dust temperature increases toward the outer regions and that these clumps are associated with the shallowest column density profiles. In contrast, molecular clumps in the protostellar or H ii region phase have dust temperature gradients more consistent with internal heating and are associated with steeper column density profiles compared with the quiescent clumps.

DOI 10.1088/0004-637X/815/2/130
Citations Scopus - 14
2015 Stephens IW, Jackson JM, Sanhueza P, Whitaker JS, Hoq S, Rathborne JM, Foster JB, 'Interferometric observations of high-mass star-forming clumps with unusual N

© 2015. The American Astronomical Society. All rights reserved.. The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey has detected high-mass star-forming clumps with anoma... [more]

© 2015. The American Astronomical Society. All rights reserved.. The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey has detected high-mass star-forming clumps with anomalous N 2 H + /HCO + (1-0) integrated intensity ratios that are either unusually high ("N 2 H + rich") or unusually low ("N 2 H + poor"). With 3 mm observations from the Australia Telescope Compact Array (ATCA), we imaged two N 2 H + rich clumps, G333.234-00.061 and G345.144-00.216, and two N 2 H + poor clumps, G351.409+00.567 and G353.229+00.672. In these clumps, the N 2 H + rich anomalies arise from extreme self-absorption of the HCO + line. G333.234-00.061 contains two of the most massive protostellar cores known with diameters of less than 0.1 pc, separated by a projected distance of only 0.12 pc. Unexpectedly, the higher mass core appears to be at an earlier evolutionary stage than the lower mass core, which may suggest that two different epochs of high-mass star formation can occur in close proximity. Through careful analysis of the ATCA observations and MALT90 clumps (including the G333, NGC 6334, and NGC 6357 star formation regions), we find that N 2 H + poor anomalies arise at clump scales and are caused by lower relative abundances of N 2 H + due to the distinct chemistry of H ii regions or photodissociation regions.

DOI 10.1088/0004-637X/802/1/6
Citations Scopus - 5
2015 Rathborne JM, Longmore SN, Jackson JM, Alves JF, Bally J, Bastian N, et al., 'A cluster in the making: ALMA reveals the initial conditions for high-mass cluster formation', Astrophysical Journal, 802 (2015)

© 2015. The American Astronomical Society. All rights reserved. G0.253+0.016 is a molecular clump that appears to be on the verge of forming a high-mass cluster: its extremely lo... [more]

© 2015. The American Astronomical Society. All rights reserved. G0.253+0.016 is a molecular clump that appears to be on the verge of forming a high-mass cluster: its extremely low dust temperature, high mass, and high density, combined with its lack of prevalent star formation, make it an excellent candidate for an Arches-like cluster in a very early stage of formation. Here we present new Atacama Large Millimeter/Sub-millimeter Array observations of its small-scale (~0.07 pc) 3 mm dust continuum and molecular line emission from 17 different species that probe a range of distinct physical and chemical conditions. The data reveal a complex network of emission features with a complicated velocity structure: there is emission on all spatial scales, the morphology of which ranges from small, compact regions to extended, filamentary structures that are seen in both emission and absorption. The dust column density is well traced by molecules with higher excitation energies and critical densities, consistent with a clump that has a denser interior. A statistical analysis supports the idea that turbulence shapes the observed gas structure within G0.253+0.016. We find a clear break in the turbulent power spectrum derived from the optically thin dust continuum emission at a spatial scale of ~0.1 pc, which may correspond to the spatial scale at which gravity has overcome the thermal pressure. We suggest that G0.253+0.016 is on the verge of forming a cluster from hierarchical, filamentary structures that arise from a highly turbulent medium. Although the stellar distribution within high-mass Arches-like clusters is compact, centrally condensed, and smooth, the observed gas distribution within G0.253+0.016 is extended, with no high-mass central concentration, and has a complex, hierarchical structure. If this clump gives rise to a high-mass cluster and its stars are formed from this initially hierarchical gas structure, then the resulting cluster must evolve into a centrally condensed structure via a dynamical process.

DOI 10.1088/0004-637X/802/2/125
Citations Scopus - 29
2015 Sakai T, Sakai N, Furuya K, Aikawa Y, Hirota T, Foster JB, et al., 'ALMA observations of the IRDC CLUMP G34.43+00.24 MM3: DNC/HNC ratio', Astrophysical Journal, 803 (2015)

© 2015. The American Astronomical Society. All rights reserved. We have observed the clump G34.43+00.24 MM3 associated with an infrared dark cloud in DNC J = 3-2, HN 13 C J = 3... [more]

© 2015. The American Astronomical Society. All rights reserved. We have observed the clump G34.43+00.24 MM3 associated with an infrared dark cloud in DNC J = 3-2, HN 13 C J = 3-2, and N 2 H + J = 3-2 with the Atacama Large Millimeter/submillimeter Array (ALMA). The N 2 H + emission is found to be relatively weak near the hot core and the outflows, and its distribution is clearly anti-correlated with the CS emission. This result indicates that a young outflow is interacting with cold ambient gas. The HN 13 C emission is compact and mostly emanates from the hot core, whereas the DNC emission is extended around the hot core. Thus, the DNC and HN 13 C emission traces warm regions near the protostar differently. The DNC emission is stronger than the HN 13 C emission toward most parts of this clump. The DNC/HNC abundance ratio averaged within a 15¿ × 15¿ area around the phase center is higher than 0.06. This ratio is much higher than the value obtained by the previous single-dish observations of DNC and HN 13 C J = 1-0 (~0.003). It seems likely that the DNC and HNC emission observed with the single-dish telescope traces lower density envelopes, while that observed with ALMA traces higher density and highly deuterated regions. We have compared the observational results with chemical-model results in order to investigate the behavior of DNC and HNC in the dense cores. Taking these results into account, we suggest that the low DNC/HNC ratio in the high-mass sources obtained by the single-dish observations are at least partly due to the low filling factor of the high density regions.

DOI 10.1088/0004-637X/803/2/70
Citations Scopus - 4
2015 Camarata MA, Jackson JM, Chambers E, 'THE NH

© 2015. The American Astronomical Society. All rights reserved. Anomalous ammonia (NH 3 ) spectra, exhibiting asymmetric hyperfine satellite intensity profiles in the () = (1, 1)... [more]

© 2015. The American Astronomical Society. All rights reserved. Anomalous ammonia (NH 3 ) spectra, exhibiting asymmetric hyperfine satellite intensity profiles in the () = (1, 1) inversion transition, have been observed in star-forming regions for over 35 years. We present a systematic study of this "hyperfine intensity anomaly" (HIA) toward a sample of 334 high-mass star forming regions: 310 high-mass (100 ) clumps and 24 infrared dark clouds. The HIA is ubiquitous in high-mass star forming regions. Although LTE excitation predicts that the intensity ratios of the outer satellites and inner satellites are exactly unity, for this sample the ensemble average ratios are 0.812 0.004 and 1.125 0.005, respectively. We have quantified the HIA and find no significant relationships between the HIA and temperature, line width, optical depth, and the stage of stellar evolution. The fact that HIAs are common in high-mass star-forming regions suggests that the conditions that lead to HIAs are ubiquitous in these regions. A possible link between HIAs and the predictions of the competitive accretion model of high-mass star formation is suggested; however, the expected trends of HIA strength with clump evolutionary stage, rotational temperature, and line width for evolving cores in competitive accretion models are not found. Thus, the exact gas structures that produce HIAs remain unknown. Turbulent gas structures are a possible explanation, but the details need to be explored.

DOI 10.1088/0004-637X/806/1/74
2015 Walker DL, Longmore SN, Bastian N, Kruijssen JMD, Rathborne JM, Jackson JM, et al., 'Tracing the conversion of gas into stars in young massive cluster progenitors', Monthly Notices of the Royal Astronomical Society, 449 715-725 (2015)

© 2015 The Authors. Whilst young massive clusters (YMCs; M ¿ 10 &lt; sup &gt; 4 &lt; /sup &gt; M¿, age ¿ 100 Myr) have been identified in significant numbers, their progenito... [more]

© 2015 The Authors. Whilst young massive clusters (YMCs; M ¿ 10 < sup > 4 < /sup > M¿, age ¿ 100 Myr) have been identified in significant numbers, their progenitor gas clouds have eluded detection. Recently, four extreme molecular clouds residing within 200 pc of the Galactic Centre have been identified as having the properties thought necessary to form YMCs. Here, we utilize far-IR continuum data from the Herschel infrared Galactic Plane Survey (HiGAL) and millimetre spectral line data from the Millimetre Astronomy Legacy Team 90GHzSurvey to determine their global physical and kinematic structure. We derive their masses, dust temperatures and radii and use virial analysis to conclude that they are all likely gravitationally bound - confirming that they are likely YMC progenitors. We then compare the density profiles of these clouds to those of the gas and stellar components of the Sagittarius B2 Main and North protoclusters and the stellar distribution of the Arches YMC. We find that even in these clouds - the most massive and dense quiescent clouds in the Galaxy - the gas is not compact enough to form an Arches-like (M = 2 × 10 < sup > 4 < /sup > M¿, R < inf > eff < /inf > = 0.4 pc) stellar distribution. Further dynamical processes would be required to condense the resultant population, indicating that the mass becomes more centrally concentrated as the (proto)cluster evolves. These results suggest that YMC formation may proceed hierarchically rather than through monolithic collapse.

DOI 10.1093/mnras/stv300
Citations Scopus - 20
2015 Camarata M, Jackson JM, Chambers E, 'Hyperfine Intensity Anomaly in High-Mass Star-Forming Regions', Astrophysical Journal, 806 74-74 (2015)
2014 Goodman AA, Alves J, Beaumont CN, Benjamin RA, Borkin MA, Burkert A, et al., 'The bones of the milky way', Astrophysical Journal, 797 (2014)

© 2014. The American Astronomical Society. All rights reserved. The very long and thin infrared dark cloud &quot;Nessie&quot; is even longer than had been previously claimed, and... [more]

© 2014. The American Astronomical Society. All rights reserved. The very long and thin infrared dark cloud "Nessie" is even longer than had been previously claimed, and an analysis of its Galactic location suggests that it lies directly in the Milky Way's mid-plane, tracing out a highly elongated bone-like feature within the prominent Scutum-Centaurus spiral arm. Re-analysis of mid-infrared imagery from the Spitzer Space Telescope shows that this infrared dark cloud (IRDC) is at least two and possibly as many as five times longer than had originally been claimed by Nessie's discoverers; its aspect ratio is therefore at least 300:1 and possibly as large as 800:1. A careful accounting for both the Sun's offset from the Galactic plane (25 pc) and the Galactic center's offset from the (l II , b II ) = (0, 0) position shows that the latitude of the true Galactic mid-plane at the 3.1 kpc distance to the Scutum-Centaurus Arm is not b = 0, but instead closer to b = -0.4, which is the latitude of Nessie to within a few parsecs. An analysis of the radial velocities of low-density (CO) and high-density (NH3) gas associated with the Nessie dust feature suggests that Nessie runs along the Scutum-Centaurus Arm in position-position-velocity space, which means it likely forms a dense "spine" of the arm in real space as well. The Scutum-Centaurus Arm is the closest major spiral arm to the Sun toward the inner Galaxy, and, at the longitude of Nessie, it is almost perpendicular to our line of sight, making Nessie the easiest feature to see as a shadow elongated along the Galactic plane from our location. Future high-resolution dust mapping and molecular line observations of the harder-to-find Galactic "bones" should allow us to exploit the Sun's position above the plane to gain a (very foreshortened) view "from above" the Milky Way's structure.

DOI 10.1088/0004-637X/797/1/53
Citations Scopus - 26
2014 Contreras Y, Rathborne J, Jackson J, Foster J, Longmore S, 'MALT90: Tracing the chemistry and kinematics of molecular clumps within the central molecular zone', Proceedings of the International Astronomical Union, 9 182-184 (2014)

The MALT90 survey targets more than 2000 high-mass star-forming clumps in the Galactic plane obtaining small maps around each of them, in 16 molecular lines at 90 GHz. By observin... [more]

The MALT90 survey targets more than 2000 high-mass star-forming clumps in the Galactic plane obtaining small maps around each of them, in 16 molecular lines at 90 GHz. By observing several thousand high-mass star forming clumps MALT90 aims to characterize their global chemical and physical evolution. Here we summarize the survey parameters and show examples of the MALT90 data toward three clumps in the central molecular zone. Copyright © International Astronomical Union 2014.

DOI 10.1017/S1743921314000507
2014 Foster JB, Arce HG, Kassis M, Sanhueza P, Jackson JM, Finn SC, et al., 'Distributed low-mass star formation in the IRDC G34.43+00.24', Astrophysical Journal, 791 (2014)

We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.... [more]

We have used deep near-infrared observations with adaptive optics to discover a distributed population of low-mass protostars within the filamentary Infrared Dark Cloud G34.43+00.24. We use maps of dust emission at multiple wavelengths to determine the column density structure of the cloud. In combination with an empirically verified model of the magnitude distribution of background stars, this column density map allows us to reliably determine overdensities of red sources that are due to embedded protostars in the cloud. We also identify protostars through their extended emission in the K band, which comes from excited H 2 in protostellar outflows or reflection nebulosity. We find a population of distributed low-mass protostars, suggesting that low-mass protostars may form earlier than, or contemporaneously with, high-mass protostars in such a filament. The low-mass protostellar population may also produce the narrow line-width SiO emission observed in some clouds without high-mass protostars. Finally, we use a molecular line map of the cloud to determine the virial parameter per unit length along the filament and find that the highest mass protostars form in the most bound portion of the filament, as suggested by theoretical models. © 2014. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/791/2/108
Citations Scopus - 13
2014 Rathborne JM, Longmore SN, Jackson JM, Foster JB, Contreras Y, Garay G, et al., 'G0.253+0.016: A centrally condensed, high-mass protocluster', Astrophysical Journal, 786 (2014)

© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. Despite their importance as stellar nurseries and the building blocks of galaxies, very litt... [more]

© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. Despite their importance as stellar nurseries and the building blocks of galaxies, very little is known about the formation of the highest mass clusters. The dense clump G0.253+0.016 represents an example of a clump that may form an Arches-like, high-mass cluster. Here we present molecular line maps toward G0.253+0.016 taken as part of theMALT90molecular line survey, complemented with APEX observations. Combined, these data reveal the global physical properties and kinematics of G0.253+0.016. Recent Herschel data show that while the dust temperature is low (~19 K) toward its center, the dust temperature on the exterior is higher (~27 K) due to external heating. Our new molecular line data reveal that, overall, the morphology of dense gas detected toward G0.253+0.016 matches its IR extinction and dust continuum emission very well. An anticorrelation between the dust and gas column densities toward its center indicates that the clump is centrally condensed with a cold, dense interior in which the molecular gas is chemically depleted. The velocity field shows a strong gradient along the clump's major axis, with the blueshifted side at a higher Galactic longitude. The optically thick gas tracers are systematically redshifted with respect to the optically thin and hot gas tracers, indicating radial motions. The gas kinematics and line ratios support the recently proposed scenario in which G0.253+0.016 results from a tidal compression during a recent pericenter passage near Sgr A*. Because G0.253+0.016 represents an excellent example of a clump that may form a high-mass cluster, its detailed study should reveal a wealth of knowledge about the early stages of cluster formation.

DOI 10.1088/0004-637X/786/2/140
Citations Scopus - 31
2014 Bally J, Rathborne JM, Longmore SN, Jackson JM, Alves JF, Bressert E, et al., 'Absorption filaments toward the massive clump G0.253+0.016', Astrophysical Journal, 795 (2014)

© 2014. The American Astronomical Society. All rights reserved. ALMA HCO + observations of the infrared dark cloud G0.253+0.016 located in the central molecular zone of the Gala... [more]

© 2014. The American Astronomical Society. All rights reserved. ALMA HCO + observations of the infrared dark cloud G0.253+0.016 located in the central molecular zone of the Galaxy are presented. The 89 GHz emission is area-filling, optically thick, and sub-thermally excited. Two types of filaments are seen in absorption against the HCO + emission. Broad-line absorption filaments (BLAs) have widths of less than a few arcseconds (0.07-0.14 pc), lengths of 30-50 arcsec (1.2-1.8 pc), and absorption profiles extending over a velocity range larger than 20 km s -1 . The BLAs are nearly parallel to the nearby G0.18 non-thermal filaments and may trace HCO + molecules gyrating about highly ordered magnetic fields located in front of G0.253+0.016 or edge-on sheets formed behind supersonic shocks propagating orthogonal to our line of sight in the foreground. Narrow-line absorption filaments (NLAs) have line widths less than 20 km s -1 . Some NLAs are also seen in absorption in other species with high optical depth, such as HCN, and occasionally in emission where the background is faint. The NLAs, which also trace low-density, sub-thermally excited HCO + molecules, are mostly seen on the blueshifted side of the emission from G0.253+0.016. If associated with the surface of G0.253+0.016, the kinematics of the NLAs indicate that the cloud surface is expanding. The decompression of entrained, milli-Gauss magnetic fields may be responsible for the re-expansion of the surface layers of G0.253+0.016 as it recedes from the Galactic center following a close encounter with Sgr A.

DOI 10.1088/0004-637X/795/1/28
Citations Scopus - 7
2014 Yanagida T, Sakai T, Hirota T, Sakai N, Foster JB, Sanhueza P, et al., 'Alma observations of the IRDC clump G34.43+00.24 MM3: 278 GHz class i methanol masers', Astrophysical Journal Letters, 794 (2014)

© 2014. The American Astronomical Society. All rights reserved. We have observed a molecular clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the CH 3 OH J K ... [more]

© 2014. The American Astronomical Society. All rights reserved. We have observed a molecular clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the CH 3 OH J K = 9 -1-8 0 E, 5 0-4 0 E, and 5 -1-4 -1 E lines at sub-arcsecond resolution by using the Atacama Large Millimeter/submillimeter Array. By comparing the CH 3 OH J K = 9 -1-8 0 E emission with the CH 3 OH 5 0-4 0 E and 5 -1-4 -1 E emission, we have found that the CH 3 OH J K = 9 -1-8 0 E emission is masing. We have clearly shown that the CH 3 OH J K = 9 -1-8 0 masers arise from the post shocked gas in the interacting regions between the outflows and ambient dense gas. Toward the strongest peak of the CH 3 OH maser emission, SiO J = 6-5 emission is very weak. This indicates that the CH 3 OH maser emission traces relatively old shocks or weak shocks.

DOI 10.1088/2041-8205/794/1/L10
Citations Scopus - 8
2014 Rathborne JM, Longmore SN, Jackson JM, Kruijssen JMD, Alves JF, Bally J, et al., 'Turbulence sets the initial conditions for star formation in high-pressure environments', Astrophysical Journal Letters, 795 (2014)

© 2014. The American Astronomical Society. All rights reserved.. Despite the simplicity of theoretical models of supersonically turbulent, isothermal media, their predictions suc... [more]

© 2014. The American Astronomical Society. All rights reserved.. Despite the simplicity of theoretical models of supersonically turbulent, isothermal media, their predictions successfully match the observed gas structure and star formation activity within low-pressure (P/k < 10 5 K cm -3 ) molecular clouds in the solar neighborhood. However, it is unknown whether or not these theories extend to clouds in high-pressure (P/k > 10 7 K cm -3 ) environments, like those in the Galaxy's inner 200 pc central molecular zone (CMZ) and in the early universe. Here, we present Atacama Large Millimeter/submillimeter Array 3 mm dust continuum emission within a cloud, G0.253+0.016, which is immersed in the high-pressure environment of the CMZ. While the log-normal shape and dispersion of its column density probability distribution function (PDF) are strikingly similar to those of solar neighborhood clouds, there is one important quantitative difference: its mean column density is one to two orders of magnitude higher. Both the similarity and difference in the PDF compared to those derived from solar neighborhood clouds match predictions of turbulent cloud models given the high-pressure environment of the CMZ. The PDF shows a small deviation from log-normal at high column densities confirming the youth of G0.253+0.016. Its lack of star formation is consistent with the theoretically predicted, environmentally dependent volume density threshold for star formation which is orders of magnitude higher than that derived for solar neighborhood clouds. Our results provide the first empirical evidence that the current theoretical understanding of molecular cloud structure derived from the solar neighborhood also holds in high-pressure environments. We therefore suggest that these theories may be applicable to understand star formation in the early universe.

DOI 10.1088/2041-8205/795/2/L25
Citations Scopus - 41
2013 Sanhueza P, Jackson JM, Foster JB, Jimenez-Serra I, Dirienzo WJ, Pillai T, 'Distinct chemical regions in the "prestellar" infrared dark cloud G028.23-00.19', Astrophysical Journal, 773 (2013)

We have observed the Infrared Dark Cloud (IRDC) G028.23-00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of th... [more]

We have observed the Infrared Dark Cloud (IRDC) G028.23-00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of the most massive (~1520 M ¿ ) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH 2D abundance, narrow molecular line widths, and absence of embedded infrared sources (from 3.6 to 70 µm) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s -1 ) and high abundances ((0.8-4) × 10 -9 ) is detected in the northern and southern regions of the IRDC, unassociated with MM1. We suggest that SiO is rele ased to the gas phase from the dust grains through shocks produced by outflows from undetected intermediate-mass stars or clusters of low-mass stars deeply embedded in the IRDC. A weaker SiO component with narrow line widths (~2 km s -1 ) and low abundances (4.3 × 10 -11 ) is detected in the center-west region, consistent with either a "subcloud-subcloud" collision or an unresolved population of a few low-mass stars. We report widespread CH 3 OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s -1 ) in a cold, massive prestellar clump (MM1). We suggest that the most likely mechanism releasing methanol into the gas phase in such a cold region is the exothermicity of grain-surface reactions. HN 13 C reveals that the IRDC is actually composed of two distinct substructures ("subclouds") separated in velocity space by ~1.4 km s -1 . The narrow SiO component arises where the subclouds overlap. The spatial distribution of C 2 H resembles that of NH 2D , which suggests that C 2H also traces cold gas in this IRDC. © 2013. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/773/2/123
Citations Scopus - 27
2013 Finn SC, Jackson JM, Rathborne JM, Chambers ET, Simon R, 'The distribution of infrared dark clouds in the first galactic quadrant', Astrophysical Journal, 764 (2013)

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetime... [more]

Infrared dark clouds (IRDCs) are believed to host the earliest stages of high-mass star and cluster formation. Because O stars typically travel short distances over their lifetimes, if IRDCs host the earliest stages of high-mass star formation then these cold, dense molecular clouds should be located in or near the spiral arms in the Galaxy. The Galactic distribution of a large sample of IRDCs should therefore provide information on Galactic structure. Moreover, determination of distances enables mass and luminosity calculations. We have observed a large sample of IRDC candidates in the first Galactic quadrant in the dense gas tracer CS (2-1) using the Mopra telescope in order to determine kinematic distances from the molecular line velocities. We find that the IRDCs are concentrated around a Galactocentric distance of ~4.5 kpc, agreeing with the results of Simon et al. This distribution is consistent with the location of the Scutum-Centaurus spiral arm. The group of IRDCs near the Sun in the first quadrant detected in 13 CO (1-0) in Simon et al. is not detected in the CS data. This discrepancy arises from the differences in the critical densities between the 13 CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13 CO (1-0) traces a population of IRDCs with lower column densities and lower 1.1 mm flux densities in addition to more dense IRDCs detected in CS. Masses of the first quadrant IRDCs are calculated from 13 CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc. © 2013. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/764/1/102
Citations Scopus - 1
2013 Hoq S, Jackson JM, Foster JB, Sanhueza P, Guzmán A, Whitaker JS, et al., 'Chemical evolution in high-mass star-forming regions: Results from the malt90 survey', Astrophysical Journal, 777 (2013)

The chemical changes of high-mass star-forming regions provide a potential method for classifying their evolutionary stages and, ultimately, ages. In this study, we search for cor... [more]

The chemical changes of high-mass star-forming regions provide a potential method for classifying their evolutionary stages and, ultimately, ages. In this study, we search for correlations between molecular abundances and the evolutionary stages of dense molecular clumps associated with high-mass star formation. We use the molecular line maps from Year 1 of the Millimetre Astronomy Legacy Team 90 GHz (MALT90) Survey. The survey mapped several hundred individual star-forming clumps chosen from the ATLASGAL survey to span the complete range of evolution, from prestellar to protostellar to H II regions. The evolutionary stage of each clump is classified using the Spitzer GLIMPSE/MIPSGAL mid-IR surveys. Where possible, we determine the dust temperatures and H 2 column densities for each clump from Herschel/Hi-GAL continuum data. From MALT90 data, we measure the integrated intensities of the N 2 H + , HCO + , HCN and HNC (1-0) lines, and derive the column densities and abundances of N 2 H + and HCO + . The Herschel dust temperatures increase as a function of the IR-based Spitzer evolutionary classification scheme, with the youngest clumps being the coldest, which gives confidence that this classification method provides a reliable way to assign evolutionary stages to clumps. Both N 2 H + and HCO + abundances increase as a function of evolutionary stage, whereas the N 2 H + (1-0) to HCO + (1-0) integrated intensity ratios show no discernable trend. The HCN (1-0) to HNC(1-0) integrated intensity ratios show marginal evidence of an increase as the clumps evolve. © 2013. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/777/2/157
Citations Scopus - 31
2013 Purcell CR, Hoare MG, Cotton WD, Lumsden SL, Urquhart JS, Chandler C, et al., 'The coordinated radio and infrared survey for high-mass star formation. II. source catalog', Astrophysical Journal, Supplement Series, 205 (2013)

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys th... [more]

The CORNISH project is the highest resolution radio continuum survey of the Galactic plane to date. It is the 5 GHz radio continuum part of a series of multi-wavelength surveys that focus on the northern GLIMPSE region (10° < l < 65°), observed by the Spitzer satellite in the mid-infrared. Observations with the Very Large Array in B and BnA configurations have yielded a 1.¿5 resolution Stokes I map with a root mean square noise level better than 0.4 mJy beam -1 . Here we describe the data-processing methods and data characteristics, and present a new, uniform catalog of compact radio emission. This includes an implementation of automatic deconvolution that provides much more reliable imaging than standard CLEANing. A rigorous investigation of the noise characteristics and reliability of source detection has been carried out. We show that the survey is optimized to detect emission on size scales up to 14¿ and for unresolved sources the catalog is more than 90% complete at a flux density of 3.9 mJy. We have detected 3062 sources above a 7s detection limit and present their ensemble properties. The catalog is highly reliable away from regions containing poorly sampled extended emission, which comprise less than 2% of the survey area. Imaging problems have been mitigated by down-weighting the shortest spacings and potential artifacts flagged via a rigorous manual inspection with reference to the Spitzer infrared data. We present images of the most common source types found: H II regions, planetary nebulae, and radio galaxies. The CORNISH data and catalog are available online at http://cornish.leeds.ac.uk. © 2013. The American Astronomical Society. All rights reserved.

DOI 10.1088/0067-0049/205/1/1
Citations Scopus - 57
2013 Foster JB, Rathborne JM, Sanhueza P, Claysmith C, Whitaker JS, Jackson JM, et al., 'Characterisation of the MALT90 survey and the mopra telescope at 90 GHz', Publications of the Astronomical Society of Australia, 30 (2013)

We characterise the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90) and the Mopra telescope at 90 GHz. We combine repeated position-switched observations of the source G30... [more]

We characterise the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90) and the Mopra telescope at 90 GHz. We combine repeated position-switched observations of the source G300.968+01.145 with a map of the same source in order to estimate the pointing reliability of the position-switched observations and, by extension, the MALT90 survey; we estimate our pointing uncertainty to be 8 arcsec. We model the two strongest sources of systematic gain variability as functions of elevation and time-of-day and quantify the remaining absolute flux uncertainty. Corrections based on these two variables reduce the scatter in repeated observations from 12%-25% down to 10%-17%. We find no evidence for intrinsic source variability in G300.968+01.145. For certain applications, the corrections described herein will be integral for improving the absolute flux calibration of MALT90 maps and other observations using the Mopra telescope at 90 GHz. © 2013 Astronomical Society of Australia.

DOI 10.1017/pasa.2013.18
Citations Scopus - 15
2013 Jackson JM, Rathborne JM, Foster JB, Whitaker JS, Sanhueza P, Claysmith C, et al., 'MALT90: The millimetre astronomy legacy team 90 GHz survey', Publications of the Astronomical Society of Australia, 30 (2013)

The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey aims to characterise the physical and chemical evolution of high-mass star-forming clumps. Exploiting the unique broad ... [more]

The Millimetre Astronomy Legacy Team 90 GHz (MALT90) survey aims to characterise the physical and chemical evolution of high-mass star-forming clumps. Exploiting the unique broad frequency range and on-the-fly mapping capabilities of the Australia Telescope National Facility Mopra 22 m single-dish telescope 1 , MALT90 has obtained 3' × 3' maps towards ~2 000 dense molecular clumps identified in the ATLASGAL 870 µm Galactic plane survey. The clumps were selected to host the early stages of high-mass star formation and to span the complete range in their evolutionary states (from prestellar, to protostellar, and on to $\mathrm{H\,{\ scriptstyle {II}}}$ regions and photodissociation regions). Because MALT90 mapped 16 lines simultaneously with excellent spatial (38 arcsec) and spectral (0.11 km s -1 ) resolution, the data reveal a wealth of information about the clumps' morphologies, chemistry, and kinematics. In this paper we outline the survey strategy, observing mode, data reduction procedure, and highlight some early science results. All MALT90 raw and processed data products are available to the community. With its unprecedented large sample of clumps, MALT90 is the largest survey of its type ever conducted and an excellent resource for identifying interesting candidates for high-resolution studies with ALMA. © 2013 Astronomical Society of Australia.

DOI 10.1017/pasa.2013.37
Citations Scopus - 38
2013 Sakai T, Sakai N, Foster JB, Sanhueza P, Jackson JM, Kassis M, et al., 'Alma observations of the irdc clump G34.43+00.24 Mm3: Hot core and molecular outflows', Astrophysical Journal Letters, 775 (2013)

We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH 3 OH, CS, 13 CS, SiO, CH 3 CH 2 CN, and HCO... [more]

We have observed a cluster forming clump (MM3) associated with the infrared dark cloud G34.43+00.24 in the 1.3 mm continuum and the CH 3 OH, CS, 13 CS, SiO, CH 3 CH 2 CN, and HCOOCH 3 lines with the Atacama Large Millimeter/submillimeter Array and in K-band with the Keck telescope. We have found a young outflow toward the center of this clump in the SiO, CS, and CH 3 OH lines. This outflow is likely driven by a protostar embedded in a hot core, which is traced by the CH 3 CH 2 CN, HCOOCH 3 , 13 CS, and high excitation CH 3 OH lines. The size of the hot core is about 800 × 300 AU in spite of its low mass ( < 1.1 M ¿ ), suggesting a high accretion rate or the presence of multiple star system harboring a few hot corinos. The outflow is highly collimated, and the dynamical timescale is estimated to be less than 740 yr. In addition, we have also detected extended emission of SiO, CS, and CH 3 OH, which is not associated with the hot core and the outflow. This emission may be related to past star formation activity in the clump. Although G34.43+00.24 MM3 is surrounded by a dark feature in infrared, it has already experienced active formation of low-mass stars in an early stage of clump evolution. © 2013. The American Astronomical Society. All rights reserved.

DOI 10.1088/2041-8205/775/1/L31
Citations Scopus - 9
2012 Longmore SN, Rathborne J, Bastian N, Alves J, Ascenso J, Bally J, et al., 'G0.253 + 0.016: A molecular cloud progenitor of an Arches-like cluster', Astrophysical Journal, 746 (2012)

Young massive clusters (YMCs) with stellar masses of 10 4 -10 5 M ¿ and core stellar densities of 10 4 -10 5 stars per cubic pc are thought to be the &quot;missing link&quot... [more]

Young massive clusters (YMCs) with stellar masses of 10 4 -10 5 M ¿ and core stellar densities of 10 4 -10 5 stars per cubic pc are thought to be the "missing link" between open clusters and extreme extragalactic super star clusters and globular clusters. As such, studying the initial conditions of YMCs offers an opportunity to test cluster formation models across the full cluster mass range. G0.253 + 0.016 is an excellent candidate YMC progenitor. We make use of existing multi-wavelength data including recently available far-IR continuum (Herschel/Herschel Infrared Galactic Plane Survey) and mm spectral line (H 2 O Southern Galactic Plane Survey and Millimetre Astronomy Legacy Team 90GHz Survey) data and present new, deep, multiple-filter, near-IR (Very Large Telescope/NACO) observations to study G0.253 + 0.016. These data show that G0.253 + 0.016 is a high-mass (1.3 × 10 5 M ¿ ), low-temperature (T dust 20K), high-volume, and column density (n 8 × 10 4 cm -3 ; cm -2 ) molecular clump which is close to virial equilibrium (M dust M virial ) so is likely to be gravitationally bound. It is almost devoid of star formation and, thus, has exactly the properties expected for the initial conditions of a clump that may form an Arches-like massive cluster. We compare the properties of G0.253 + 0.016 to typical Galactic cluster-forming molecular clumps and find it is extreme, and possibly unique in the Galaxy. This uniqueness makes detailed studies of G0.253 + 0.016 extremely important for testing massive cluster formation models. © 2012. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/746/2/117
Citations Scopus - 83
2012 Hoare MG, Purcell CR, Churchwell EB, Diamond P, Cotton WD, Chandler CJ, et al., 'The coordinated radio and infrared survey for High-mass star formation (The CORNISH Survey). I. Survey design', Publications of the Astronomical Society of the Pacific, 124 939-955 (2012)

We describe the motivation, design, and implementation of the CORNISH survey, an arcsecondresolution radio continuum survey of the inner galactic plane at 5 GHz using the Very Lar... [more]

We describe the motivation, design, and implementation of the CORNISH survey, an arcsecondresolution radio continuum survey of the inner galactic plane at 5 GHz using the Very Large Array (VLA). It is a blind survey coordinated with the northern Spitzer GLIMPSE I region covering 10° < l < 65° and |b| < 1° at similar resolution. We discuss in detail the strategy that we employed to control the shape of the synthesised beam across this survey, which covers a wide range of fairly low declinations. Two snapshots separated by 4 h kept the beam elongation to less that 1.5 over 75% of the survey area and less than 2 over 98% of the survey. The prime scientific motivation is to provide an unbiased survey for ultra-compact H II regions to study this key phase in massive star formation. A sensitivity around 2 mJy will allow the automatic distinction between radio-loud and radio-quiet mid- IR sources found in the Spitzer surveys. This survey has many legacy applications beyond star formation, including evolved stars, active stars and binaries, and extragalactic sources. The CORNISH survey for compact ionized sources complements other Galactic plane surveys that target diffuse and nonthermal sources, as well as atomic and molecular phases to build up a complete picture of the interstellar medium in the Galaxy. © 2012. The Astronomical Society of the Pacific.

DOI 10.1086/668058
Citations Scopus - 54
2012 Sandstrom KM, Bolatto AD, Bot C, Draine BT, Ingalls JG, Israel FP, et al., 'The spitzer spectroscopic survey of the small magellanic cloud (S

We present results of mid-infrared spectroscopic mapping observations of six star-forming regions in the Small Magellanic Cloud (SMC) from the Spitzer Spectroscopic Survey of the ... [more]

We present results of mid-infrared spectroscopic mapping observations of six star-forming regions in the Small Magellanic Cloud (SMC) from the Spitzer Spectroscopic Survey of the SMC (S 4 MC). We detect the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) in all of the mapped regions, greatly increasing the range of environments where PAHs have been spectroscopically detected in the SMC. We investigate the variations of the mid-IR bands in each region and compare our results to studies of the PAH bands in the SINGS sample and in a sample of low-metallicity starburst galaxies. PAH emission in the SMC is characterized by low ratios of the 6-9 µm features relative to the 11.3 µm feature and weak 8.6 and 17.0 µm features. Interpreting these band ratios in the light of laboratory and theoretical studies, we find that PAHs in the SMC tend to be smaller and less ionized than those in higher metallicity galaxies. Based on studies of PAH destruction, we argue that a size distribution shifted toward smaller PAHs cannot be the result of processing in the interstellar medium, but instead reflects differences in the formation of PAHs at low metallicity. Finally, we discuss the implications of our observations for our understanding of the PAH life-cycle in low-metallicity galaxies - namely that the observed deficit of PAHs may be a consequence of PAHs forming with smaller average sizes and therefore being more susceptible to destruction under typical interstellar medium conditions.

DOI 10.1088/0004-637X/744/1/20
Citations Scopus - 30
2012 Foster JB, Stead JJ, Benjamin RA, Hoare MG, Jackson JM, 'Distances to dark clouds: Comparing extinction distances to maser parallax distances', Astrophysical Journal, 751 (2012)

We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near-infrared (Two Micron All ... [more]

We test two different methods of using near-infrared extinction to estimate distances to dark clouds in the first quadrant of the Galaxy using large near-infrared (Two Micron All Sky Survey and UKIRT Infrared Deep Sky Survey) surveys. Very long baseline interferometry parallax measurements of masers around massive young stars provide the most direct and bias-free measurement of the distance to these dark clouds. We compare the extinction distance estimates to these maser parallax distances. We also compare these distances to kinematic distances, including recent re-calibrations of the Galactic rotation curve. The extinction distance methods agree with the maser parallax distances (within the errors) between 66% and 100% of the time (depending on method and input survey) and between 85% and 100% of the time outside of the crowded Galactic center. Although the sample size is small, extinction distance methods reproduce maser parallax distances better than kinematic distances; furthermore, extinction distance methods do not suffer from the kinematic distance ambiguity. This validation gives us confidence that these extinction methods may be extended to additional dark clouds where maser parallaxes are not available. © 2012. The American Astronomical Society. All rights reserved..

DOI 10.1088/0004-637X/751/2/157
Citations Scopus - 21
2012 Sanhueza P, Jackson JM, Foster JB, Garay G, Silva A, Finn SC, 'Chemistry in infrared dark cloud clumps: A molecular line survey at 3mm', Astrophysical Journal, 756 (2012)

We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3mm using the 22m ATNF Mopra Telescope located in Australi... [more]

We have observed 37 Infrared Dark Clouds (IRDCs), containing a total of 159 clumps, in high-density molecular tracers at 3mm using the 22m ATNF Mopra Telescope located in Australia. After determining kinematic distances, we eliminated clumps that are not located in IRDCs and clumps with a separation between them of less than one Mopra beam. Our final sample consists of 92 IRDC clumps. The most commonly detected molecular lines are (detection rates higher than 8%) N 2 H + , HNC, HN 13 C, HCO + , H 13 CO + , HCN, C 2 H, HC 3 N, HNCO, and SiO. We investigate the behavior of the different molecular tracers and look for chemical variations as a function of an evolutionary sequence based on Spitzer IRAC and MIPS emission. We find that the molecular tracers behave differently through the evolutionary sequence and some of them can be used to yield useful relative age information. The presence of HNC and N 2 H + lines does not depend on the star formation activity. On the other hand, HC 3 N, HNCO, and SiO are predominantly detected in later stages of evolution. Optical depth calculations show that in IRDC clumps the N 2 H + line is optically thin, the C 2 H line is moderately optically thick, and HNC and HCO + are optically thick. The HCN hyperfine transitions are blended, and, in addition, show self-absorbed line profiles and extended wing emission. These factors combined prevent the use of HCN hyperfine transitions for the calculation of physical parameters. Total column densities of the different molecules, except C 2 H, increase with the evolutionary stage of the clumps. Molecular abundances increase with the evolutionary stage for N 2 H + and HCO + . The N 2 H + /HCO + and N 2 H + /HNC abundance ratios act as chemical clocks, increasing with the evolution of the clumps. © © 2012. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/756/1/60
Citations Scopus - 57
2011 Roman-Duval J, Federrath C, Brunt C, Heyer M, Jackson J, Klessen RS, 'The turbulence spectrum of molecular clouds in the galactic ring survey: A density-dependent principal component analysis calibration', Astrophysical Journal, 740 (2011)

Turbulence plays a major role in the formation and evolution of molecular clouds. Observationally, turbulent velocities are convolved with the density of an observed region. To co... [more]

Turbulence plays a major role in the formation and evolution of molecular clouds. Observationally, turbulent velocities are convolved with the density of an observed region. To correct for this convolution, we investigate the relation between the turbulence spectrum of model clouds, and the statistics of their synthetic observations obtained from principal component analysis (PCA). We apply PCA to spectral maps generated from simulated density and velocity fields, obtained from hydrodynamic simulations of supersonic turbulence, and from fractional Brownian motion (fBm) fields with varying velocity, density spectra, and density dispersion. We examine the dependence of the slope of the PCA pseudo-structure function, a PCA , on intermittency, on the turbulence velocity (ß v ) and density (ß n ) spectral indexes, and on density dispersion. We find that PCA is insensitive to ß n and to the log-density dispersion s s , provided s s = 2. For s s > 2, a PCA increases with s s due to the intermittent sampling of the velocity field by the density field. The PCA calibration also depends on intermittency. We derive a PCA calibration based on fBm structures with s s = 2 and apply it to 367 13 CO spectral maps of molecular clouds in the Galactic Ring Survey. The average slope of the PCA structure function, <a PCA > = 0.62 0.2, is consistent with the hydrodynamic simulations and leads to a turbulence velocity exponent of <ß v > = 2.06 0.6 for a non-intermittent, low density dispersion flow. Accounting for intermittency and density dispersion, the coincidence between the PCA slope of the GRS clouds and the hydrodynamic simulations suggests ß v ¿ 1.9, consistent with both Burgers and compressible intermittent turbulence. © 2011. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/740/2/120
Citations Scopus - 45
2011 Roman-Duval J, Federrath C, Brunt C, Heyer M, Jackson JM, Klessen R, 'The Turbulence Spectrum of Molecular Clouds in the Galactic Ring Survey: A Density-Dependent PCA Calibration', Astrophysical Journal, 740 120-120 (2011)
2011 Rathborne JM, Garay G, Jackson JM, Longmore S, Zhang Q, Simon R, 'Hot molecular cores in Infrared Dark Clouds', Astrophysical Journal, 741 (2011)

We present high angular resolution continuum images and molecular line spectra obtained at 345GHz with the Submillimeter Array (SMA) toward two massive cores that lie within Infra... [more]

We present high angular resolution continuum images and molecular line spectra obtained at 345GHz with the Submillimeter Array (SMA) toward two massive cores that lie within Infrared Dark Clouds (IRDCs): G034.43+00.24 MM1 and G024.33+00.11 MM1. Both of these cores contain bright, unresolved ( < 2¿) objects that have previously been imaged in the millimeter/submillimeter continuum with the Institut de RadioAstronomie Millimétrique (IRAM) Plateau de Bure Interferometer and SMA and show complex molecular line chemistry. The new, higher angular resolution SMA continuum images reveal that both cores contain massive (8, 26 M ¿ ), unresolved (0¿6; ~3000AU) continuum emission features and emission from many complex molecular transitions, which confirm that these are hot molecular cores, an early stage in the formation of a high-mass star. Because these hot cores are located within IRDCs, they may well represent the very earliest phases in the formation of high-mass protostars and, hence, their detailed study may reveal the initial conditions within high-mass star-forming cores, before they are shredded apart by stellar winds and radiation. © 2011. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/741/2/120
Citations Scopus - 13
2011 Foster JB, Jackson JM, Barnes PJ, Barris E, Brooks K, Cunningham M, et al., 'The Millimeter Astronomy Legacy Team 90GHz (MALT90) pilot survey', Astrophysical Journal, Supplement Series, 197 (2011)

We describe a pilot survey conducted with the Mopra 22m radio telescope in preparation for the Millimeter Astronomy Legacy Team Survey at 90GHz (MALT90). We identified 182 candida... [more]

We describe a pilot survey conducted with the Mopra 22m radio telescope in preparation for the Millimeter Astronomy Legacy Team Survey at 90GHz (MALT90). We identified 182 candidate dense molecular clumps using six different selection criteria and mapped each source simultaneously in 16 different lines near 90GHz. We present a summary of the data and describe how the results of the pilot survey shaped the design of the larger MALT90 survey. We motivate our selection of target sources for the main survey based on the pilot detection rates and demonstrate the value of mapping in multiple lines simultaneously at high spectral resolution. © 2011. The American Astronomical Society. All rights reserved.

DOI 10.1088/0067-0049/197/2/25
Citations Scopus - 52
2010 Vernazza P, Carry B, Emery J, Hora JL, Cruikshank D, Binzel RP, et al., 'Mid-infrared spectral variability for compositionally similar asteroids: Implications for asteroid particle size distributions', Icarus, 207 800-809 (2010)

We report an unexpected variability among mid-infrared spectra (IRTF and Spitzer data) of eight S-type asteroids for which all other remote sensing interpretations (e.g. VNIR spec... [more]

We report an unexpected variability among mid-infrared spectra (IRTF and Spitzer data) of eight S-type asteroids for which all other remote sensing interpretations (e.g. VNIR spectroscopy, albedo) yield similar compositions. Compositional fitting making use of their mid-IR spectra only yields surprising alternative conclusions: (1) these objects are not " compositionally similar" as the inferred abundances of their main surface minerals (olivine and pyroxene) differ from one another by 35% and (2) carbonaceous chondrite and ordinary chondrite meteorites provide an equally good match to each asteroid spectrum.Following the laboratory work of Ramsey and Christensen (Ramsey, M.S., Christensen, P.R. [1998]. J. Geophys. Res. 103, 577-596), we interpret this variability to be physically caused by differences in surface particle size and/or the effect of space weathering processes. Our results suggest that the observed asteroids must be covered with very fine ( < 5µm) dust that masks some major and most minor spectral features. We speculate that the compositional analysis may be improved with a spectral library containing a wide variety of well characterized spectra (e.g., olivine, orthopyroxene, feldspar, iron, etc.) obtained from very fine powders. In addition to the grain size effect, space weathering processes may contribute as well to the reduction of the spectral contrast. This can be directly tested via new laboratory irradiation experiments. © 2010 Elsevier Inc.

DOI 10.1016/j.icarus.2010.01.011
Citations Scopus - 23
2010 Rathborne JM, Jackson JM, Chambers ET, Stojimirovic I, Simon R, Shipman R, Frieswijk W, 'The early stages of star formation in infrared dark clouds: Characterizing the core dust properties', Astrophysical Journal, 715 310-322 (2010)

Identified as extinction features against the bright Galactic mid-infrared background, infrared dark clouds (IRDCs) are thought to harbor the very earliest stages of star and clus... [more]

Identified as extinction features against the bright Galactic mid-infrared background, infrared dark clouds (IRDCs) are thought to harbor the very earliest stages of star and cluster formation. In order to better characterize the properties of their embedded cores, we have obtained new 24µm, 60-100µm, and submillimeter continuum data toward a sample of 38 IRDCs. The 24µmSpitzerimages reveal that while the IRDCs remain dark, many of the cores are associated with bright 24µmemission sources, which suggests that they contain one or more embedded protostars. Combining the 24µm, 60-100µm, and submillimeter continuum data, we have constructed broadband spectral energy distributions (SEDs) for 157 of the cores within these IRDCs and, using simple graybody fits to the SEDs, have estimated their dust temperatures, emissivities, opacities, bolometric luminosities, masses, and densities. Based on their Spitzer/Infrared Array Camera 3-8µmcolors and the presence of 24µmpoint-source emission, we have separated cores that harbor active, high-mass star formation from cores that are quiescent. The active "protostellar" cores typically have warmer dust temperatures and higher bolometric luminosities than the more quiescent, perhaps "pre-protostellar," cores. Because the mass distributions of the populations are similar, however, we speculate that the active and quiescent cores may represent different evolutionary stages of the same underlying population of cores. Although we cannot rule out low-mass star formation in the quiescent cores, the most massive of them are excellent candidates for the "high-mass starless core" phase, the very earliest in the formation of a high-mass star. © 2010. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/715/1/310
Citations Scopus - 70
2010 Battersby C, Bally J, Jackson JM, Ginsburg A, Shirley YL, Schlingman W, Glenn J, 'An infrared through radio study of the properties and evolution of IRDC clumps', Astrophysical Journal, 721 222-250 (2010)

We examine the physical properties and evolutionary stages of a sample of 17 clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared, millimeter, and radio dat... [more]

We examine the physical properties and evolutionary stages of a sample of 17 clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared, millimeter, and radio data with new Bolocam Galactic Plane Survey (BGPS) 1.1 mm data, Very Large Array radio continuum data, and Heinrich Hertz Telescope dense gas (HCO + and N 2 H + ) spectroscopic data. We combine literature studies of star formation tracers and dust temperatures within IRDCs with our search for ultracompact (UC) HII regions to discuss a possible evolutionary sequence for IRDC clumps. In addition, we perform an analysis of mass tracers in IRDCs and find that 8µm extinction masses and 1.1 mm BGPS masses are complementary mass tracers in IRDCs except for the most active clumps (notably those containing UC Hii regions), for which both mass tracers suffer biases. We find that the measured virial masses in IRDC clumps are uniformly higher than the measured dust continuum masses on the scale of ~1 pc. We use 13 CO, HCO + , and N 2 H+ to study the molecular gas properties of IRDCs and do not see any evidence of chemical differentiation between hot and cold clumps on the scale of ~1 pc. However, both HCO + and N 2 H + are brighter in active clumps, due to an increase in temperature and/or density. We report the identification of four UC HII regions embedded within IRDC clumps and find that UC Hii regions are associated with bright (¿1 Jy) 24µm point sources, and that the brightest UC HII regions are associated with "diffuse red clumps" (an extended enhancement at 8µm). The broad stages of the discussed evolutionary sequence (from a quiescent clump to an embedded HII region) are supported by literature dust temperature estimates; however, no sequential nature can be inferred between the individual star formatio n tracers. © 2010. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/721/1/222
Citations Scopus - 53
2010 Roman-Duval J, Jackson JM, Heyer M, Rathborne J, Simon R, 'Physical properties and galactic distribution of molecular clouds identified in the galactic ring survey', Astrophysical Journal, 723 492-507 (2010)

We derive the physical properties of 580 molecular clouds based on their 12 CO and 13 CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galacti... [more]

We derive the physical properties of 580 molecular clouds based on their 12 CO and 13 CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of molecular clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the interstellar medium is around 2.36. This relation, M = (228 ± 18) R 2.36±0.04 , allows us to derive masses for an additional 170 Galactic Ring Survey (GRS) molecular clouds not covered by the UMSB survey.We derive the Galactic surface mass density of molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in molecular gas, the Galactic surface density of molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that molecular clouds form in spiral arms and are disrupted in the inter-arm space. Finally, we find that the CO excitation temperature of molecular clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of molecular clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm. © 2010. The American Astronomical Society.

DOI 10.1088/0004-637X/723/1/492
Citations Scopus - 147
2010 Jackson JM, Finn SC, Chambers ET, Rathborne JM, Simon R, 'The "nessie" Nebula: Cluster formation in a filamentary infrared dark cloud', Astrophysical Journal Letters, 719 (2010)

The &quot;Nessie&quot; Nebula is a filamentary infrared dark cloud (IRDC) with a large aspect ratio of over 150:1 (1. ? 5 × 0. ? 01 or 80 pc × 0.5 pc at a kinematic distance of ... [more]

The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large aspect ratio of over 150:1 (1. ? 5 × 0. ? 01 or 80 pc × 0.5 pc at a kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense molecular gas, made with the Australia Telescope National Facility Mopra telescope, show an excellent morphological match to the mid-IR extinction. Moreover, because the molecular line emission from the entire nebula has the same radial velocity to within ±3.4 kms - 1 , the nebula is a single, coherent cloud and not the chance alignment of multiple unrelated clouds along the line of sight. The Nessie Nebula contains a number of compact, dense molecular cores which have a characteristic projected spacing of ~4.5 pc along the filament. The theory of gravitationally bound gaseous cylinders predicts the existence of such cores, which, due to the "sausage" or "varicose" fluid instability, fragment from the cylinder at a characteristic length scale. If turbulent pressure dominates over thermal pressure in Nessie, then the observed core spacing matches theoretical predictions. We speculate that the formation of highmass stars and massive star clusters arises from the fragmentation of filamentary IRDCs caused by the "sausage" fluid instability that leads to the formation of massive, dense molecular cores. The filamentary molecular gas clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334, etc.) may represent a later stage of IRDC evolution. © 2010. The American Astronomical Society. All rights reserved.

DOI 10.1088/2041-8205/719/2/L185
Citations Scopus - 72
2009 Matthews H, Kirk H, Johnstone D, Weferling B, Cohen M, Jenness T, et al., 'Looking through the galactic plane: Imaging cold dust toward l = 44°', Astronomical Journal, 138 1380-1402 (2009)

We present imaging observations of continuum emission from interstellar dust at 850 and 1200 µm of a section of the Galactic Plane covering 2 deg 2 centered at l = 44°. Complem... [more]

We present imaging observations of continuum emission from interstellar dust at 850 and 1200 µm of a section of the Galactic Plane covering 2 deg 2 centered at l = 44°. Complementary jiggle-mapping and fast-scanning techniques were used, respectively, at these two wavelengths. The mapped area includes the well-known star formation regions W49 and G45.1/45.5. Using an automated clump-finding routine, we identify 132 compact 850 µm emission features within the region above a completeness level of about 200 mJy beam -1 . The positions of the latter objects were used to determine fluxes from the 1200 µm image. Spectral line data were subsequently obtained with the same observing beamwidth as at 850 µm for almost half of the objects; these were either imaged in the 13 CO (3-2) line, or basic characteristics determined using the 12 CO (3-2) transition. We use these data, supplemented by existing 13 CO (1-0) and HI survey data, to determine distances and hence derive masses for the dust clump ensemble, assuming a uniform dust temperature of 15 K. From these data we find that the number-mass relationship for clumps in the field is similar to that found for individual star-forming regions. © 2009 The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-6256/138/5/1380
Citations Scopus - 10
2009 Heyer M, Krawczyk C, Duval J, Jackson JM, 'Re-examining larson's scaling relationships in galactic molecular clouds', Astrophysical Journal, 699 1092-1103 (2009)

The properties of Galactic molecular clouds tabulated by Solomon et al. (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of 13 CO J = 1-0 emission. T... [more]

The properties of Galactic molecular clouds tabulated by Solomon et al. (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of 13 CO J = 1-0 emission. These new data provide a lower opacity tracer of molecular clouds and improved angular and spectral resolution compared with previous surveys of molecular line emission along the Galactic Plane. We calculate giant molecular cloud (GMC) masses within the SRBY cloud boundaries assuming local thermodynamic equilibrium (LTE) conditions throughout the cloud and a constant H 2 to 13 CO abundance, while accounting for the variation of the 12 C 13 C with galactocentric radius. The LTE-derived masses are typically five times smaller than the SRBY virial masses. The corresponding median mass surface density of molecular hydrogen for this sample is 42 M pc -2 , which is significantly lower than the value derived by SRBY (median 206 M pc -2 ) that has been widely adopted by most models of cloud evolution and star formation. This discrepancy arises from both the extrapolation by SRBY of velocity dispersion, size, and CO luminosity to the 1 K antenna temperature isophote that likely overestimates the GMC masses and our assumption of constant 13 CO abundance over the projected area of each cloud. Owing to the uncertainty of molecular abundances in the envelopes of clouds, the mass surface density of GMCs could be larger than the values derived from our 13 CO measurements. From velocity dispersions derived from the 13 CO data, we find that the coefficient of the cloud structure functions, v ° = s v R 1/2 , is not constant, as required to satisfy Larson's scaling relationships, but rather systematically varies with the surface density of the cloud as S 0.5 as expected for clouds in self-gravitational equilibrium. © 2009. The American Astronomical Society.

DOI 10.1088/0004-637X/699/2/1092
Citations Scopus - 250
2009 Roman-Duval J, Jackson JM, Heyer M, Johnson A, Rathborne J, Shah R, Simon R, 'Kinematic distances to molecular clouds identified in the galactic ring survey', Astrophysical Journal, 699 1153-1170 (2009)

Kinematic distances to 750 molecular clouds identified in the 13 CO J = 1 ¿ 0 Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS) are derived a... [more]

Kinematic distances to 750 molecular clouds identified in the 13 CO J = 1 ¿ 0 Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS) are derived assuming the Clemens rotation curve of the Galaxy. The kinematic distance ambiguity is resolved by examining the presence of H I self-absorption toward the 13 CO emission peak of each cloud using the Very Large Array Galactic Plane Survey. We also identify 21 cm continuum sources embedded in the GRS clouds in order to use absorption features in the H I 21 cm continuum to distinguish between near and far kinematic distances. The Galactic distribution of GRS clouds is consistent with a four-arm model of the Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS clouds match star-count data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire star-count data. We conclude that molecular clouds must form in spiral arms and be short-lived (lifetimes < 10 7 yr) in order to explain the absence of massive, 13 CO bright molecular clouds in the interarm space. © 2009. The American Astronomical Society. All rights reserved.

DOI 10.1088/0004-637X/699/2/1153
Citations Scopus - 108
2009 Rathborne JM, Jackson JM, Simon R, Zhang Q, 'Infrared dark clouds as precursors to star clusters', Astrophysics and Space Science, 324 155-162 (2009)

Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2mm continuum emi... [more]

Infrared dark clouds (IRDCs) are cold, dense molecular clouds identified as extinction features against the bright mid-infrared Galactic background. Our recent 1.2mm continuum emission survey of IRDCs reveals many compact ( < 0.5 pc) and massive (10-2100 M¿) cores within them. These prestellar cores hold the key to understanding IRDCs and their role in star formation. Here, we present high angular resolution spectral-line and mm/sub-mm continuum images obtained with the IRAM Plateau de Bure Interferometer and the Sub-Millimeter Array towards three high-mass IRDC cores. The high angular resolution images reveal that two of the cores are resolved into multiple, compact protostellar condensations, while the remaining core contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that it is a hot molecular core. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar, while two of the cores are also forming lower-mass protostars. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters. © Springer Science+Business Media B.V. 2009.

DOI 10.1007/s10509-009-0121-8
Citations Scopus - 4
2009 Smith N, Whitney BA, Conti PS, De Pree CG, Jackson JM, 'Massive star formation and feedback in W49A: The source of our Galaxy's most luminous water maser outflow', Monthly Notices of the Royal Astronomical Society, 399 952-965 (2009)

We present high spatial resolution mid-infrared (mid-IR) images of the ring of ultracompact H ii regions in W49A obtained at Gemini North, allowing us to identify the driving sour... [more]

We present high spatial resolution mid-infrared (mid-IR) images of the ring of ultracompact H ii regions in W49A obtained at Gemini North, allowing us to identify the driving source of its powerful H 2 O maser outflow. These data also confirm our previous report that several radio sources in the ring are undetected in the mid-IR because they are embedded deep inside the cloud core. We locate the source of the water maser outflow at the position of the compact mid-IR peak of source G (source G:IRS1) to within 0.07 arcsec. This IR source is not coincident with any identified compact radio continuum source, but is coincident with a hot molecular core, so we propose that G:IRS1 is a hot core driving an outflow analogous to the wide-angle bipolar outflow in OMC-1. G:IRS1 is at the origin of a larger bipolar cavity and CO outflow. The water maser outflow is orthogonal to the bipolar CO cavity, so the masers probably reside near its waist in the thin cavity walls. Models of the IR emission require a massive protostar with M * ¿ 45 M ¿ , L * ¿ 3 × 10 5 L ¿ and an effective envelope accretion rate of ~10 -3 M ¿ yr -1 . Feedback from the central star could potentially drive the small-scale H 2 O maser outflow, but it has insufficient radiative momentum to have driven the large-scale bipolar CO outflow, requiring that this massive star had an active accretion disc over the past 10 4 yr. Combined with the spatially resolved morphology in IR images, G:IRS1 in W49 provides compelling evidence for a massive protostar that formed by accreting from a disc, accompanied by a bipolar outflow. © 2009 RAS.

DOI 10.1111/j.1365-2966.2009.15343.x
Citations Scopus - 12
2009 Rathborne JM, Johnson AM, Jackson JM, Shah RY, Simon R, 'Molecular clouds and clumps in the boston university-five college radio astronomy observatory galactic ring survey', Astrophysical Journal, Supplement Series, 182 131-142 (2009)

The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS) of 13 CO J = 1 ¿ 0 emission covers Galactic longitudes 18° &lt; l &lt; ... [more]

The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS) of 13 CO J = 1 ¿ 0 emission covers Galactic longitudes 18° < l < 557 and Galactic latitudes |b| = 1°. Using the SEQUOIA array on the FCRAO 14 m telescope, the GRS fully sampled the 13 CO Galactic emission (46¿ angular resolution on a 22¿ grid) and achieved a spectral resolution of 0.21 km s -1 . Because the GRS uses 13 CO, an optically thin tracer, rather than 12 CO, an optically thick tracer, the GRS allows a much better determination of column density and also a cleaner separation of velocity components along a line of sight. With this homogeneous, fully sampled survey of 13 CO emission, we have identified 829 molecular clouds and 6124 clumps throughout the inner Galaxy using the CLUMPFIND algorithm. Here we present details of the catalog and a preliminary analysis of the properties of the molecular clouds and their clumps. Moreover, we compare clouds inside and outside of the 5 kpc ring and find that clouds within the ring typically have warmer temperatures, higher column densities, larger areas, and more clumps compared with clouds located outside the ring. This is expected if these clouds are actively forming stars. This catalog provides a useful tool for the study of molecular clouds and their embedded young stellar objects. © 2009. The American Astronomical Society. All rights reserved.

DOI 10.1088/0067-0049/182/1/131
Citations Scopus - 62
2009 Anderson LD, Bania TM, Jackson JM, Clemens DP, Heyer M, Simon R, et al., 'The molecular properties of galactic h II regions', Astrophysical Journal, Supplement Series, 181 255-271 (2009)

We derive the molecular properties for a sample of 301 Galactic H n regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within th... [more]

We derive the molecular properties for a sample of 301 Galactic H n regions including 123 ultra compact (UC), 105 compact, and 73 diffuse nebulae. We analyze all sources within the BU-FCRAO Galactic Ring Survey (GRS) of 13 CO emission known to be H n regions based upon the presence of radio continuum and cm-wavelength radio recombination line emission. Unlike all previous large area coverage 13 CO surveys, the GRS is fully sampled in angle and yet covers ~ 75 deg 2 of the Inner Galaxy. The angular resolution of the GRS (46¿) allows us to associate molecular gas with H n regions without ambiguity and to investigate the physical properties of this molecular gas. We find clear CO/H II morphological associations in position and velocity for ~ 80% of the nebular sample. Compact H II region molecular gas clouds are on average larger than UC clouds: 2.'2 compared to 1.'7. Compact and UC H H regions have very similar molecular properties, with ~ 5 K line intensities and ~ 4 km s -1 line widths. The diffuse H n region molecular gas has lower line intensities, ~ 3 K, and smaller line widths, ~ 3.5 km s -1 . These latter characteristics are similar to those found for quiescent molecular clouds in the GRS. Our sample nebulae thus show evidence for an evolutionary sequence wherein small, dense molecular gas clumps associated with UC H II regions grow into older compact nebulae and finally fragment and dissipate into large, diffuse nebulae. © 2009. The American Astronomical Society. All rights reserved.

DOI 10.1088/0067-0049/181/1/255
Citations Scopus - 34
2009 Chambers ET, Jackson JM, Rathborne JM, Simon R, 'Star formation activity of cores within infrared dark clouds', Astrophysical Journal, Supplement Series, 181 360-390 (2009)

Infrared Dark Clouds (IRDCs) contain compact cores which probably host the early stages of high-mass star formation. Many of these cores contain regions of extended, enhanced 4.5 ... [more]

Infrared Dark Clouds (IRDCs) contain compact cores which probably host the early stages of high-mass star formation. Many of these cores contain regions of extended, enhanced 4.5 µm emission, the so-called "green fuzzies," which indicate shocked gas. Many cores also contain 24 µm emission, presumably from heated dust which indicates embedded protostars. Because "green fuzzies" and 24 µm point sources both indicate star formation, we have developed an algorithm to identify star-forming cores within IRDCs by searching for the simultaneous presence of these two distinct indicators. We employ this algorithm on a sample of 190 cores found toward IRDCs, and classify the cores as "active" if they contain a green fuzzy coincident with an embedded 24 µm source, and as "quiescent" if they contain neither IR signature. We hypothesize that the "quiescent" cores represent the earliest "preprotostellar" (starless) core phase, before the development of a warm protostar, and that the "active" cores represent a later phase, after the development of a protostar. We test this idea by comparing the sizes, densities, and maser activity of the "active" and "quiescent" cores. We find that, on average, "active" cores have smaller sizes, higher densities, and more pronounced water and methanol maser activity than the "quiescent" cores. This is expected if the "quiescent" cores are in an earlier evolutionary state than the "active" cores. The masses of "active" cores suggest that they may be forming highmass stars. The highest mass "quiescent" cores are excellent candidates for the elusive high-mass starless cores. © 2009. The American Astronomical Society. All rights reserved.

DOI 10.1088/0067-0049/181/2/360
Citations Scopus - 131
2008 Jackson JM, Finn SC, Rathborne JM, Chambers ET, Simon R, 'The galactic distribution of infrared dark clouds', Astrophysical Journal, 680 349-361 (2008)

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in or... [more]

CS (2-1) measurements toward a large sample of fourth Galactic quadrant infrared dark clouds (IRDCs) were made with the Australia Telescope National Facility Mopra telescope in order to establish their kinematic distances and Galactic distribution. Due to its large critical density, CS unambiguously separates the dense IRDCs from more diffuse giant molecular clouds. The fourth-quadrant IRDCs show a pronounced peak in their radial galactocentric distribution at R = 6 kpc. The first-quadrant IRDC distribution (traced by 13 CO emission) also shows a peak, but at a galactocentric radius of R = 5 kpc rather than 6 kpc. This disparity in the peak galactocentric radius suggests that IRDCs trace a spiral arm which lies closer to the Sun in the fourth quadrant. Indeed, the deduced IRDC distribution matches the location of the Scutum-Centaurus arm in Milky Way models dominated by two spiral arms. Since, in external galaxies, OB stars form primarily in spiral arms, the association of IRDCs with a Milky Way spiral ami supports the idea that high-mass stars form in. IRDCs. The first-quadrant IRDC distribution also reveals a second peak near the solar circle, possibly due to the fact that 13 CO could trace somewhat lower density IRDCs. The reliability of the MSX IRDC catalog by Simon and coworkers is estimated by using the CS detection rate of IRDC candidates. The overall reliability is at least 58%, and increases to near 100% for high contrasts, Galactic longitudes within ~30° of the Galactic center, and large mid-IR backgrounds. A significant fraction of our IRDC sample (14%) showed two CS velocity components, which probably represent two distinct IRDCs along the saine line of sight. © 2008. The American Astronomical Society. All rights reserved.

DOI 10.1086/587539
Citations Scopus - 56
2008 Rathborne JM, Jackson JM, Zhang Q, Simon R, 'Submillimeter Array observations of infrared dark clouds: A tale of two cores', Astrophysical Journal, 689 1141-1149 (2008)

We present high angular resolution submillimeter continuum images and molecular line spectra obtained with the Submillimeter Array toward two massive cores that lie within infrare... [more]

We present high angular resolution submillimeter continuum images and molecular line spectra obtained with the Submillimeter Array toward two massive cores that lie within infrared dark clouds (IRDCs), one actively star-forming (G034.43+00.24 MM1) and the other more quiescent (G028.43-00.25 MM1). The high angular resolution submillimeter continuum image of G034.43+00.24 MM1 reveals a compact (~0.03 pc) and massive (~29 M {bull's eye} ) structure, while the molecular line spectrum shows emission from numerous complex molecules. Such a rich molecular line spectrum from a compact region indicates that G034.43+00.24 MM 1 contains a hot molecular core, an early stage in the formation of a high-mass protostar. Moreover, the velocity structure of its 13 CO (3-2) emission indicates that this BO protostar may be surrounded by a rotating circumstellar envelope. In contrast, the submillimeter continuum image of G028.53-00.25 MM1 reveals three compact (¿0.06 pc), massive (9-21 {bull's eye} ) condensations, but there are no lines detected in its spectrum. We suggest that the core G028.53-00.25 MM1 is in a very early stage in the high-mass star formation process; its size and mass are sufficient to form at least one high-mass star, yet it shows no signs of localized heating. Because the combination of high-velocity line wings with a large IR-to-millimeterbolometric luminosity (~10 2 L {bull's eye} ) indicates that this core has already begun to form accreting protostars, we speculate that the condensations may be in the early phase of accretion and may eventually become high-mass protostars. Therefore, we have found the possible existence of two high-mass star-forming cores: one in a very early phase of star formation and one in the later hot-core phase. Together, the properties of these two cores support the idea that the earliest stages of high-mass star formation occur within IRDCs. © 2008. The American Astronomical Society. All rights reserved.

DOI 10.1086/592733
Citations Scopus - 50
2007 Bolatto AD, Simon JD, Stanimirovic S, Van Loon JT, Shah RY, Venn K, et al., 'The Spitzer survey of the small magellanic cloud: S

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands.... [more]

We present the initial results from the Spitzer Survey of the Small Magellanic Cloud (S 3 MC), which imaged the star-forming body of the SMC in all seven MIPS and IRAC wave bands. We find that the F 8 /F 24 ratio (an estimate of PAH abundance) has large spatial variations and takes a wide range of values that are unrelated to metallicity but anticorrelated with 24 µm brightness and F 24 /F 70 ratio. This suggests that photodestruction is primarily responsible for the low abundance of PAHs observed in star-forming low-metallicity galaxies. We use the S 3 MC images to compile a photometric catalog of ~400,000 mid- and far-infrared point sources in the SMC. The sources detected at the longest wavelengths fall into four main categories: (1) bright 5.8 µm sources with very faint optical counterparts and very red mid-infrared colors ([5.8] -[8.0] > 1.2), which we identify as YSOs; (2) bright mid-infrared sources with mildly red colors (0.16 = [5.8] -[8.0] < 0.6), identified as carbon stars; (3) bright mid-infrared sources with neutral colors and bright optical counterparts, corresponding to oxygen-rich evolved stars; and (4) unreddened early B stars (B3-O9) with a large 24 µm excess. This excess is reminiscent of debris disks and is detected in only a small fraction of these stars (=5%). The majority of the brightest infrared point sources in the SMC fall into groups 1-3. We use this photometric information to produce a catalog of 282 bright YSOs in the SMC with a very low level of contamination (~7%). © 2007. The American Astronomical Society. All rights reserved.

DOI 10.1086/509104
Citations Scopus - 138
2007 Mercer EP, Clemens DP, Rathborne JM, Meade MR, Babler BL, Indebetouw R, et al., 'A GLIMPSE of the Southern Jellyfish Nebula and its massive YSO', Astrophysical Journal, 656 242-247 (2007)

In Spitzer/IRAC images obtained under the GLIMPSE Legacy Survey, we have identified a unique and provocative nebular object we call the &quot;Southern Jellyfish Nebula.&quot; The ... [more]

In Spitzer/IRAC images obtained under the GLIMPSE Legacy Survey, we have identified a unique and provocative nebular object we call the "Southern Jellyfish Nebula." The Southern Jellyfish Nebula is characterized by a fan of narrow tendrils with extreme length-to-width ratios that emanate from the vicinity of a bright infrared point source embedded in a smaller resolved nebula. From CO observations of the Nebula's morphologically associated molecular cloud, we have derived a kinematic distance of 5.7 ± 0.8 kpc and a cloud mass of 3.2 ± 0.9 × 10 3 M ¿ . The tendril-like ropes of the Nebula have widths of ~0.1 pc and lengths of up to ~2 pc. We have integrated the infrared spectral energy distribution (SED) of the point source to establish it as a massive young stellar object (MYSO), most likely forming alone, but possibly masking fainter cluster members. The shape of the SED is consistent with the shape of a late Class 0 SED model. Based on its far-IR luminosity of 3.3 ± 0.9 × 10 4 L ¿ , the Southern Jellyfish's MYSO has a zero-age main sequence (ZAMS) spectral type of B0. Given the curious nature of this nebula, we suspect its peculiar IR-bright structure is directly related to its current state of star formation. © 2007. The American Astronomical Society. All rights reserved.

DOI 10.1086/510302
Citations Scopus - 4
2007 Uzpen B, Kobulnicky HA, Monson AJ, Pierce MJ, Clemens DP, Backman DE, et al., 'The frequency of mid-infrared excess sources in galactic surveys', Astrophysical Journal, 658 1264-1288 (2007)

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 µm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB star... [more]

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 µm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main-sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-IR K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional IR luminosities in the range 191 K < T < 787 K and 3.9 × 10 -4 < L IR /L* < 2.7 × 10 -1 . The majority of our modeled sample (14 stars) have 10 -3 < L IR /L* < 10 -2 and are consistent with either transition disks or massive debris disks. These objects have fractional IR luminosities and temperatures between those of ß Pic-type debris disk systems (L IR /L* = 10 -3 ) and Class II pre-main-sequence systems (L IR /L* × 10 -1 ). We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0% ±0.3%. © 2007. The American Astronomical Society. All rights reserved.

DOI 10.1086/511736
Citations Scopus - 11
2007 Rathborne JM, Simon R, Jackson JM, 'The detection of protostellar condensations in infrared dark cloud cores', Astrophysical Journal, 662 1082-1092 (2007)

Infrared dark clouds (IRDCs) are a distinct class of interstellar molecular cloud identified as dark extinction features against the bright mid-infrared Galactic background. Here ... [more]

Infrared dark clouds (IRDCs) are a distinct class of interstellar molecular cloud identified as dark extinction features against the bright mid-infrared Galactic background. Here we present high angular resolution millimeter continuum images obtained with the IRAM Plateau de Bure Interferometer toward four high-mass (200-1800 M ¿ ) IRDC cores that show evidence for active high-mass star formation (M > 8 M ¿ ). We detect twelve bright ( > 7 mJy), compact (¿2¿, ¿0.024 pc) condensations toward these cores. Two of the cores (G024.60+00.08 MM1 and G024.60+00.08 MM2) are resolved into multiple protostellar condensations, while one core (G022.35+00.41 MM1) shows two condensations. The remaining core (G024.33+00.11 MM1) contains a single, compact protostellar condensation with a very rich molecular spectrum, indicating that this is a hot molecular core associated with an early stage in the formation of a high-mass protostar. The derived gas masses for these condensations suggest that each core is forming at least one high-mass protostar (M gas > 8 M ¿ ), and three cores are also forming lower mass protostars (M gas ~ 2-5 M ¿ ). A comparison of the ratios of the gas masses (M G ) to the Jeans masses (M J ) for IRDCs, cores, and condensations, provides broad support for the idea of hierarchical fragmentation. The close proximity of multiple protostars of disparate mass indicates that these IRDCs are in the earliest evolutionary states in the formation of stellar clusters. © 2007. The American Astronomical Society. All rights reserved.

DOI 10.1086/513178
Citations Scopus - 68
2006 Wang Y, Zhang Q, Rathborne JM, Jackson J, Wu Y, 'Water masers associated with infrared dark cloud cores', Astrophysical Journal, 651 (2006)

We present a survey of the 6 16 -5 23 H 2 O maser transition toward a sample of 140 compact cores in infrared dark clouds using the Very Large Array. Strong ( &gt; 1 Jy) H 2 O ma... [more]

We present a survey of the 6 16 -5 23 H 2 O maser transition toward a sample of 140 compact cores in infrared dark clouds using the Very Large Array. Strong ( > 1 Jy) H 2 O maser emission was found associated with 17 cores, indicative of star formation in these cores. We infer that the cores with H 2 O masers have embedded protostars. Cores associated with maser emission have masses of 12 to 2 × 10 3 M ¿ , similar to the mass range in the entire sample. The H 2 O maser detection rate (12%) toward the compact, cold cores is much lower than that toward high-mass protostellar objects and ultracompact H II regions. The detection rate of H 2 O masers is significantly higher for higher mass cores than for lower mass cores. We suggest that some of the most massive infrared dark cloud cores without H 2 O maser emission are at an evolutionary stage earlier than the protostellar phases. They are prime candidates for high-mass starless cores. © 2006. The American Astronomical Society. All rights reserved.

Citations Scopus - 53
2006 Kassis M, Adams JD, Campbell MF, Deutsch LK, Hora JL, Jackson JM, Tollestrup EV, 'Mid-infrared emission at photodissociation regions in the orion nebula', Astrophysical Journal, 637 823-837 (2006)

The mid-infrared emission from a photodissociation region (PDR) viewed edge-on in the Orion Nebula is examined through 8.7-20.6 µm images and 8-13 µm spectra. The polycyclic aro... [more]

The mid-infrared emission from a photodissociation region (PDR) viewed edge-on in the Orion Nebula is examined through 8.7-20.6 µm images and 8-13 µm spectra. The polycyclic aromatic hydrocarbon (PAH) emission is located between the edges of H II regions and layers of [C I] emission, agreeing with PDR theory. Using a simple model, the spatial variations in the emission from PAHs detected at 8.6, 11.2, and 12.7 µm are demonstrated to be directly proportional to the material column density and the intensity of the UV field. For a homogeneous, neutral cloud illuminated by a bright OB star, PDR theory predicts that the ultraviolet (UV) radiation is attenuated exponentially (e -1.8Av ). The predicted UV attenuation is confirmed by observations of broad PAH emission features found at 8.6, 11.2, and 12.7 µm. The PAH emission is found in cool regions having greater optical depths relative to regions where mid-infrared emission from ionized gas is observed. Through modeling we determine a gas density of 9.7 × 104 cm -3 . On large and small size scales, the relative strengths of the 8.6,11.2, and 12.7 µm PAH features at the bar of the Orion Nebula indicate that there is not a simple transition from ionized to neutral PAHs across the PDR. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/498404
Citations Scopus - 20
2006 Simon R, Jackson JM, Rathborne JM, Chambers ET, 'A catalog of Midcourse Space Experiment infrared dark cloud candidates', Astrophysical Journal, 639 227-236 (2006)

We use 8.3 µm mid-infrared images acquired with the Midcourse Space Experiment satellite to identify and catalog infrared dark clouds (IRDCs) in the first and fourth quadrants of... [more]

We use 8.3 µm mid-infrared images acquired with the Midcourse Space Experiment satellite to identify and catalog infrared dark clouds (IRDCs) in the first and fourth quadrants of the Galactic plane. Because IRDCs are seen as dark extinction features against the diffuse Galactic infrared background, we identify them by first determining a model background from the 8.3 µm images and then searching for regions of high decremental contrast with respect to this background. IRDC candidates in our catalog are defined by contiguous regions bounded by closed contours of a 2 s decremental contrast threshold. Although most of the identified IRDCs are actual cold dark clouds, some as yet unknown fraction may be spurious identifications. For large high-contrast clouds, we estimate the reliability to be 82%. Low-contrast clouds should have lower reliabilities. Verification of the reality of individual clouds will require additional data. We identify 10,931 candidate IRDCs. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the cloud's average extinction, are found by iteratively fitting two-dimensional elliptical Gaussian functions to the contrast peaks. We identify 12,774 cores. The catalog contains the position, angular size, orientation, area, peak contrast, peak contrast signal-to-noise, and integrated contrast of the candidate IRDCs and their cores. The distribution of IRDCs closely follows the Galactic diffuse mid-infrared background and peaks toward prominent star-forming regions, spiral arm tangents, and the so-called 5 kpc Galactic molecular ring. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/499342
Citations Scopus - 146
2006 Rathborne JM, Jackson JM, Simon R, 'Infrared dark clouds: Precursors to star clusters', Astrophysical Journal, 641 389-405 (2006)

Infrared dark clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs ... [more]

Infrared dark clouds (IRDCs) are dense molecular clouds seen as extinction features against the bright mid-infrared Galactic background. Millimeter continuum maps toward 38 IRDCs reveal extended cold dust emission to be associated with each of the IRDCs. IRDCs range in morphology from filamentary to compact and have masses of 120 to 16,000 M ¿ , with a median mass of ~940 M ¿ . Each IRDC contains at least one compact (=0.5 pc) dust core and most show multiple cores. We find 140 cold millimeter cores unassociated with MSX8 µm. emission. The core masses range from 10 to 2100 M ¿ , with a median mass of ~120 M ¿ . The slope of the IRDC core mass spectrum (a ~ 2.1 ± 0.4) is similar to that of the stellar IMF. Assuming that each core will form a single star, the majority of the cores will form OB stars. IRDC cores have similar sizes, masses, and densities as hot cores associated with individual, young high-mass stars, but they are much colder. We therefore suggest that IRDC represent an earlier evolutionary phase in high-mass star formation. In addition, because IRDCs contain many compact cores and have the same sizes and masses as molecular clumps associated with young clusters, we suggest that IRDCs are the cold precursors to star clusters. Indeed, an estimate of the star formation rate within molecular clumps with similar properties to IRDCs (~2 M ¿ yr -1 ) is comparable to the global star formation rate in the Galaxy, supporting the idea mat all stars may form in such clumps. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/500423
Citations Scopus - 268
2006 Simon R, Rathborne JM, Shah RY, Jackson JM, Chambers ET, 'The characterization and galactic distribution of infrared dark clouds', Astrophysical Journal, 653 1325-1335 (2006)

Using 13 CO J = 1 ¿ 0 molecular line emission from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinema... [more]

Using 13 CO J = 1 ¿ 0 molecular line emission from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinematic distances to 313 infrared dark clouds (IRDCs) by matching the morphology of the molecular line emission in distinct velocity channels to their mid-infrared extinction. The Galactic distribution of IRDCs shows an enhancement toward the Galaxy's most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of ~5 pc, peak column densities of ~10 22 cm -2 , LIE masses of ~5 × 10 3 M ¿ , and volume-averaged H 2 densities of ~2 × 10 3 cm -3 . The similarity of these properties to those of molecular clumps associated with active star formation suggests that IRDCs represent the densest clumps within giant molecular clouds where clusters may eventually form. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/508915
Citations Scopus - 133
2006 Churchwell E, Povich MS, Allen D, Taylor MG, Meade MR, Babler BL, et al., 'The bubbling Galactic disk', Astrophysical Journal, 649 759-778 (2006)

A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent ... [more]

A visual examination of the images from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) has revealed 322 partial and closed rings that we propose represent partially or fully enclosed three-dimensional bubbles. We argue that the bubbles are primarily formed by hot young stars in massive star formation regions. We have found an average of about 1.5 bubbles per square degree. About 25% of the bubbles coincide with known radio H II regions, and about 13% enclose known star clusters. It appears that B4-B9 stars (too cool to produce detectable radio H II regions) probably produce about three-quarters of the bubbles in our sample, and the remainder are produced by young O-B3 stars that produce detectable radio H II regions. Some of the bubbles may be the outer edges of H II regions where PAH spectral features are excited and may not be dynamically formed by stellar winds. Only three of the bubbles are identified as known SNRs. No bubbles coincide with known planetary nebulae or W-R stars in the GLIMPSE survey area. The bubbles are small. The distribution of angular diameters peaks between 1' and 3' with over 98% having angular diameters less than 10' and 88% less than 4'. Almost 90% have shell thicknesses between 0.2 and 0.4 of their outer radii. Bubble shell thickness increases approximately linearly with shell radius. The eccentricities are rather large, peaking between 0.6 and 0.7; about 65% have eccentricities between 0.55 and 0.85. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/507015
Citations Scopus - 247
2006 Jackson JM, Rathborne JM, Shah RY, Simon R, Bania TM, Clemens DP, et al., 'The Boston University - Five College Radio Astronomy Observatory Galactic Ring Survey', Astrophysical Journal, Supplement Series, 163 145-159 (2006)

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13 CO J = 1 ¿ 0 emission. The survey used the SEQUOIA multipixel ... [more]

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13 CO J = 1 ¿ 0 emission. The survey used the SEQUOIA multipixel array on the Five College Radio Astronomy Observatory 14 m telescope to cover a longitude range of l= 18°-55°7 and a latitude range of |b| < 1°, a total of 75.4 deg 2 . Using both position-switching and On-The-Fly mapping modes, we achieved an angular sampling of 22¿, better than half of the telescope's 46¿ angular resolution. The survey's velocity coverage is -5 to 135 km s -1 for Galactic longitudes l = 40° and -5 to 85 km s -1 for Galactic longitudes l > 40°. At the velocity resolution of 0.21 km s -1 , the typical rms sensitivity is s(T A *) ~ 0.13 K. The survey comprises a total of 1,993,522 spectra. We show integrated intensity images (zeroth moment maps), channel maps, position-velocity diagrams, and an average spectrum of the completed survey data set. We also discuss the telescope and instrumental parameters, the observing modes, the data reduction processes, and the emission and noise characteristics of the data set. The Galactic Ring Survey data are available to the community online or in DVD form by request. © 2006. The American Astronomical Society. All rights reserved.

DOI 10.1086/500091
Citations Scopus - 312
2005 O'Dea CP, Gallimore J, Stanghellini C, Baum SA, Jackson JM, 'A search for molecular gas in GHz-peaked spectrum radio sources', Astronomical Journal, 129 610-614 (2005)

We present searches for molecular gas (CO, OH, CS, and NH 3 ) in six gigahertz-peaked spectrum (GPS) radio sources. We do not detect gas in any source and place upper limits on t... [more]

We present searches for molecular gas (CO, OH, CS, and NH 3 ) in six gigahertz-peaked spectrum (GPS) radio sources. We do not detect gas in any source and place upper limits on the mass of molecular gas that are generally in the range from ~10 9 to a few times 10 10 M¿. These limits are consistent with the following interpretations: (1) GPS sources do not require very dense gas in their hosts, and (2) the GPS sources are unlikely to be confined by dense gas and will evolve to become larger radio sources. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/427133
Citations Scopus - 11
2005 Obulnicky HAK, Monson AJ, Buckalew BA, Darnel JM, Uzpen B, Meade MR, et al., 'Discovery of a new low-latitude milky way globular cluster using glimpse', Astronomical Journal, 129 239-250 (2005)

Spitzer Space Telescope imaging from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) reveals a previously unidentified low-latitude rich star cluster near l... [more]

Spitzer Space Telescope imaging from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) reveals a previously unidentified low-latitude rich star cluster near l = 31°.3, b = -0°.1. Near-infrared JHK' photometry from the Wyoming Infrared Observatory indicates an extinction of A v ¿ 15 ± 3 mag for cluster members. Analysis of 13 CO features along the same sight line suggests a probable kinematic distance of 3.1-5.2 kpc. The new cluster has an angular diameter of ~ 1-2 pc, a total magnitude corrected for extinction of m K0 = 2.1, and a luminosity of M K ¿ -10.3 at 3.1 kpc. In contrast to young massive Galactic clusters with ages less than 100 Myr, the new cluster has no significant radio emission. Comparison with theoretical K-band luminosity functions indicates an age of at least several gigayears and a mass of at least 10 5 M¿. Unlike known old open clusters, this new cluster lies in the inner Galaxy at R GC ¿ 6.1 kpc. We designate this object "GLIMPSE-C01" and present evidence that it is a Milky Way globular cluster passing through the Galactic disk. We also identify a region of star formation and fan-shaped outflows from young stellar objects in the same field as the cluster. The cluster's passage through the Galactic molecular layer may have triggered this star formation activity. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/426337
Citations Scopus - 32
2005 Indebetouw R, Mathis JS, Babler BL, Meade MR, Watson C, Whitney BA, et al., 'The wavelength dependence of interstellar extinction from 1.25 to 8.0 µm using glimpse data', Astrophysical Journal, 619 931-938 (2005)

We determine and tabulate A [¿] / A K , the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 µm = ¿ = 8.0 µm along two lines of sight: l = 42°... [more]

We determine and tabulate A [¿] / A K , the wavelength dependence of interstellar extinction, in the Galactic plane for 1.25 µm = ¿ = 8.0 µm along two lines of sight: l = 42° and 284°. The first is a relatively quiescent and unremarkable region; the second contains the giant H II region RCW 49, as well as a "field" region unrelated to the cluster and nebulosity. Areas near these Galactic longitudes were imaged at J, H, and K bands by 2MASS and at 3-8 µm by Spitzer for the GLIMPSE Legacy program. We measure the mean values of the color excess ratios (A [¿] - A K )/(A J - A K ) directly from the color distributions of observed stars. The extinction ratio between two of the filters, e.g., A J /A K , is required to calculate A [¿] /A K from those measured ratios. We use the apparent JHK magnitudes of giant stars along our two sight lines and fit the reddening as a function of magnitude (distance) to determine A J kpc -1 , A K kpc -1 , and A J /A K . Our values of A [¿] /A K show a flattening across the 3-8 µm wavelength range, roughly consistent with the extinction measurements derived by Lutz and coworkers for the sight line toward the Galactic center. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/426679
Citations Scopus - 441
2005 Bradford CM, Stacey GJ, Nikola T, Bolatto AD, Jackson JM, Savage ML, Davidson JA, 'Warm molecular gas traced with CO J = 1 ¿ 6 in the galaxy's central 2 parsecs: Dynamical heating of the circumnuclear disk', Astrophysical Journal, 623 866-876 (2005)

We present an 11¿ resolution map of the central 2 pc of the Galaxy in the CO J = 7 ¿ 6 rotational transition. The CO emission shows rotation about Sgr A* but also evidence for n... [more]

We present an 11¿ resolution map of the central 2 pc of the Galaxy in the CO J = 7 ¿ 6 rotational transition. The CO emission shows rotation about Sgr A* but also evidence for noncircular turbulent motion and a clumpy morphology. We combine our data set with available CO measurements to model the physical conditions in the disk. We find that the molecular gas in the region is both warm and dense, with T ~ 200-300 K and n H2 ~ (5-7) × 10 4 cm -3 . The mass of warm molecular gas we measure in the central 2 pc is at least 2000 M ¿ , about 20 times the UV-excited atomic gas mass, ruling out a UV heating scenario for the molecular material. We compare the available spectral tracers with theoretical models and conclude that molecular gas is heated with magnetohydrodynamic shocks with v ~ 10-20 km s -1 and B ~ 0.3-0.5 mG. Using the conditions derived with the CO analysis, we include the other important coolants, neutral oxygen and molecular hydrogen, to estimate the total cooling budget of the molecular material. We derive a mass-to-luminosity ratio of ~2-3 M ¿ L ¿ -1 , which is consistent with the total power dissipated via turbulent decay in 0.1 pc cells with v rms ~ 15 km s -1 . These size and velocity scales are comparable to the observed clumping scale and the velocity dispersion. At this rate, the material near Sgr A* is dissipating its orbital energy on an orbital timescale and cannot last for more than a few orbits. Our conclusions support a scenario in which the features near Sgr A* such as the circumnuclear disk and northern arm are generated by infalling clouds with low specific angular momentum. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/428659
Citations Scopus - 37
2005 Rathborne JM, Jackson JM, Chambers ET, Simon R, Shipman R, Frieswijk W, 'Massive protostars in the infrared dark cloud MSXDC G034.43+00.24', Astrophysical Journal, 630 (2005)

We present a multiwavelength study of the infrared dark cloud MSXDC G034.43+00.24. Dust emission, traced by millimeter/submmillimeter images obtained with the IRAM, JCMT, and CSO ... [more]

We present a multiwavelength study of the infrared dark cloud MSXDC G034.43+00.24. Dust emission, traced by millimeter/submmillimeter images obtained with the IRAM, JCMT, and CSO telescopes, reveals three compact cores within this infrared dark cloud with masses of 170-800 M¿ and sizes < 0.5 pc. Spitzer 3.6-8.0 µm images show slightly extended emission toward these cores, with a spectral enhancement at 4.5 µm that probably arises from shocked H 2 . In addition, the broad line widths (¿V ~ 10 km s -1 ) of HCN (4-3) and CS (3-2) and the detection of SiO (2-1), observed with the JCMT and IRAM telescopes, also indicate active star formation. Spitzer 24 µm images reveal that each of these cores contains a bright, unresolved continuum source; these sources are most likely embedded protostars. Their millimeter-to-mid-IR continuum spectral energy distributions reveal very high luminosities, 9000-32,000 L¿. Because such large luminosities cannot arise from low-mass protostars, MSXDC G034.43+00.24 is actively forming massive (~10 M¿) stars. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/491656
Citations Scopus - 83
2005 Benjamin RA, Churchwell E, Babler BL, Indebetouw R, Meade MR, Whitney BA, et al., 'First glimpse results on the stellar structure of the galaxy', Astrophysical Journal, 630 (2005)

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10° = |l| = 65° and |b| = 1°, wa... [more]

The GLIMPSE (Galactic Legacy Mid-Plane Survey Extraordinaire) Point Source Catalog of ~30 million mid-infrared sources toward the inner Galaxy, 10° = |l| = 65° and |b| = 1°, was used to determine the distribution of stars in Galactic longitude, l, latitude, b, and apparent magnitude, m. The counts versus longitude can be approximated by the modified Bessel function N = N 0 (l/l 0 ) K 1 ,(l/l 0 ), where l 0 is insensitive to limiting magnitude, band choice, and side of Galactic center: l 0 = 17°-30° with a best-fit value in the 4.5 µm band of l 0 = 24° ± 4°. Modeling the source distribution as an exponential disk yields a radial scale length of H* = 3.9 ± 0.6 kpc. There is a pronounced north-south asymmetry in source counts for |l| ¿ 30°, with ~25% more stars in the north. For l = 10°-30°, there is a strong enhancement of stars of m = 11.5-13.5 mag. A linear bar passing through the Galactic center with half-length R bar = 4.4 ± 0.5 kpc, tilted by f = 44° ± 10° to the Sun-Galactic center line, provides the simplest interpretation of these data. We examine the possibility that enhanced source counts at l = 26°-28°, 31°.5-34°, and 306°-309° are related to Galactic spiral structure. Total source counts are depressed in regions where the counts of red objects (m K - m [8.0] > 3) peak. In these areas, the counts are reduced by extinction due to molecular gas, high diffuse backgrounds associated with star formation, or both. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/491785
Citations Scopus - 227
2005 Uzpen B, Kobulnicky HA, Olsen KAG, Clemens DP, Laurance TL, Meade MR, et al., 'Identification of main-sequence stars with mid-infrared excesses using glimpse: ß pictoris analogs?', Astrophysical Journal, 629 512-525 (2005)

Spitzer IRAC 3.6-8 µm photometry obtained as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral types in a 1.2 deg 2 field cent... [more]

Spitzer IRAC 3.6-8 µm photometry obtained as part of the GLIMPSE survey has revealed mid-infrared excesses for 33 field stars with known spectral types in a 1.2 deg 2 field centered on the southern Galactic H II region RCW 49. These stars comprise a subset of 184 stars with known spectral classification, most of which were preselected to have unusually red IR colors. We propose that the mid-IR excesses are caused by circumstellar dust disks that are either very late remnants of stellar formation or debris disks generated by planet formation. Of these 33 stars, 29 appear to be main-sequence stars on the basis of optical spectral classifications. Five of the 29 main-sequence stars are O or B stars with excesses that can be plausibly explained by thermal bremsstrahlung emission, and four are post-main-sequence stars. The lone O star is an O4 V((f)) at a spectrophotometric distance of 3233 -535 +540 pc and may be the earliest member of the Westerlund 2 cluster. Of the remaining 24 main-sequence stars, 18 have spectral energy distributions that are consistent with hot dusty debris disks, a possible signature of planet formation. Modeling the excesses as blackbodies demonstrates that the blackbody components have fractional bolometric disk-to-star luminosity ratios, L IR /L*, ranging from 10 -3 to 10 -2 with temperatures ranging from 220 to 820 K. The inferred temperatures are more consistent with asteroid belts than with the cooler temperatures expected for Kuiper belts. Mid-IR excesses are found in all spectral types from late B to early K. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/431479
Citations Scopus - 22
2005 Stanimirovic S, Bolatto AD, Sandstrom K, Leroy AK, Simon JD, Gaensler BM, et al., 'Spitzer space telescope detection of the Young supernova remnant 1E 0102.2-7219', Astrophysical Journal, 632 (2005)

We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The r... [more]

We present infrared observations of the young, oxygen-rich supernova remnant 1E 0102.2-7219 (E0102) in the Small Magellanic Cloud, obtained with the Spitzer Space Telescope. The remnant is detected at 24 µm but not at 8 or 70 µm and has a filled morphology with two prominent filaments. We find evidence for the existence of up to 8 × 10 -4 M ¿ of hot dust (T d ~ 120 K) associated with the remnant. Most of the hot dust is located in the central region of E0102, which appears significantly enhanced in infrared and radio continuum emission relative to the X-ray emission. Even if all of the hot dust was formed in the explosion of E0102, the estimated mass of dust is at least 100 times lower than what is predicted by some recent theoretical models. © 2005. The American Astronomical Society. All rights reserved.

DOI 10.1086/497985
Citations Scopus - 24
2005 Mercer EP, Clemens DP, Meade MR, Babler BL, Indebetouw R, Whiney BA, et al., 'New star clusters discovered in the GLIMPSE survey', Astrophysical Journal, 635 560-569 (2005)

A systematic and automated search of the extensive GLIMPSE mid-infrared survey data of the inner Galaxy was carried out to uncover new star clusters. This search has yielded 59 ne... [more]

A systematic and automated search of the extensive GLIMPSE mid-infrared survey data of the inner Galaxy was carried out to uncover new star clusters. This search has yielded 59 new clusters. Using our automated search algorithm, these clusters were identified as significant localized overdensities in the GLIMPSE point-source catalog (GLMC) and archive (GLMA). Subsequent visual inspection of the GLIMPSE image mosaics confirmed the existence of these clusters plus an additional 33 heavily embedded clusters missed by our detection algorithm, for a total of 92 newly discovered clusters. These previously uncataloged clusters range in type from heavily embedded to fully exposed clusters. More than half of the clusters have memberships exceeding 35 stars, and nearly all the clusters have diameters of 3' or less. The Galactic latitude distribution of the clusters reveals that the majority are concentrated toward the Galactic midplane. There is an asymmetry in the number of clusters located above and below the midplane, with more clusters detected below the midplane. We also observe an asymmetry in the number of clusters detected in the northern and southern halves of the Galaxy, with more than twice as many clusters detected in the south. © 2005. The American Astronomical Society, All rights reserved.

DOI 10.1086/497260
Citations Scopus - 99
2004 Paglione TAD, Yam O, Tosaki T, Jackson JM, 'The structure, kinematics, and physical properties of the molecular gas in the starburst nucleus of NGC 253', Astrophysical Journal, 611 835-845 (2004)

We present 5¿.2 x 2¿.6 resolution interferometry of CO J = 1 ¿ 0 emission from the starburst galaxy NGC 253. The high spatial resolution of these new data, in combination with ... [more]

We present 5¿.2 x 2¿.6 resolution interferometry of CO J = 1 ¿ 0 emission from the starburst galaxy NGC 253. The high spatial resolution of these new data, in combination with recent high-resolution maps of 13 CO, HCN, and near-infrared emission, allow us for the first time to link unambiguously the gas properties in the central starburst of NGC 253 with its bar dynamics. We confirm that the star formation results from bar-driven gas flows as seen in "twin peaks" galaxies. Two distinct kinematic features are evident from the CO map and position-velocity diagram: a group of clouds rotating as a solid body about the kinematic center of the galaxy and a more extended gas component associated with the near-infrared bar. We model the line intensities of CO, HCN, and 13 CO to infer the physical conditions of the gas in the nucleus of NGC 253. The results indicate increased volume densities around the radio nucleus in a twin peaks morphology. Compared with the CO kinematics, the gas densities appear highest near the radius of a likely inner Linblad resonance and slightly lead the bar minor axis. This result is similar to observations of the face-on, twin peaks galaxy NGC 6951 and is consistent with models of starburst generation due to gas inflow along a bar.

DOI 10.1086/422354
Citations Scopus - 21
2004 Churchwell E, Whitney BA, Babler BL, Indebetouw R, Meade MR, Watson C, et al., 'RCW 49 at mid-infrared wavelengths: A glimpse from the Spitzer Space Telescope', Astrophysical Journal, Supplement Series, 154 322-327 (2004)

The luminous, massive star formation region RCW 49, located in the southern Galactic plane, was imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope as part... [more]

The luminous, massive star formation region RCW 49, located in the southern Galactic plane, was imaged with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope as part of the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) program. The IRAC bands contain polycyclic aromatic hydrocarbon (PAH) features at 3.3, 6.2, 7.7, and 8.6 µm, as well as the Bra line. These features are the major contributors to the diffuse emission from RCW 49 in the IRAC bands. The Spitzer IRAC images show that the dust in RCW 49 is distributed in a network of fine filaments, pillars, knots, sharply defined boundaries, bubbles, and bow shocks. The regions immediately surrounding the ionizing star cluster and W-R stars are evacuated of dust by stellar winds and radiation. The IRAC images of RCW 49 suggest that the dust in RCW 49 has been sculpted by the winds and radiation from the embedded luminous stars in the inner 5' (inner ~6 pc) of the nebula. At projected angular radii f > 5' from the central ionizing cluster, the azimuthally averaged infrared intensity falls off as ~f -3 . Both high-resolution radio and mid-IR images suggest that the nebula is density bounded along its western boundary. The filamentary structure of the dust in RCW 49 suggests that the nebula has a small dust filling factor and, as a consequence, the entire nebula may be slightly density bounded to H-ionizing photons.

DOI 10.1086/422504
Citations Scopus - 79
2004 Whitney BA, Indebetouw R, Babler BL, Meade MR, Watson C, Wolff MJ, et al., 'A glimpse of star formation in the giant H II region RCW 49', Astrophysical Journal, Supplement Series, 154 315-321 (2004)

GLIMPSE imaging using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicates that star formation is ongoing in the RCW 9 giant H II region. A photometric compar... [more]

GLIMPSE imaging using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope indicates that star formation is ongoing in the RCW 9 giant H II region. A photometric comparison of the sources in RCW 49 to a similar area to its north finds that at least 300 stars brighter than 13th magnitude in band [3.6] have infrared excesses inconsistent with reddening due to foreground extinction. These are likely young stellar objects (YSOs) more massive than 2.5 M ¿ , suggesting that thousands more low-mass stars are forming in this cloud. Some of the YSOs are massive (B stars) and therefore very young, suggesting that a new generation of star formation is occurring, possibly triggered by stellar winds and shocks generated by the older (2-3 Myr) central massive cluster. The Spitzer IRAC camera has proven to be ideally suited for distinguishing young stars from field stars, and the GLIMPSE survey of the Galactic plane will likely find thousands of new star formation regions.

DOI 10.1086/422557
Citations Scopus - 61
2004 Mercer EP, Clemens DP, Bania TM, Jackson JM, Rathborne JM, Shah RY, et al., 'Discovery of a distant star formation region using glimpse', Astrophysical Journal, Supplement Series, 154 328-332 (2004)

Examination of early, in-orbit checkout (IOC) images of a portion of the Galactic plane obtained by the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope revealed th... [more]

Examination of early, in-orbit checkout (IOC) images of a portion of the Galactic plane obtained by the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope revealed the presence of an extended emission nebula with internal structure. The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) data show this nebula, located at l ~ 42° and b ~ 0.5°, contains bright point sources and two nonstellar regions. Ancillary data sets were used to help reveal the nature of this nebula and its exciting objects. In particular, 13 CO J = 1 ¿ 0 line emission mapped by the Galactic Ring Survey (GRS) shows molecular gas associated with the infrared nebula. The 13 CO radial velocity yields a far-kinematic distance of 11.1 kpc to the nebula, since there is no evidence for H I self-absorption. At 11.1 kpc, the far-infrared luminosity of the nebula is 4.8 × 10 4 L ¿ , and the mass of its molecular cloud is 1.1 × 10 -4 M ¿ . The spectral energy distribution rises steeply from 2.2 to 100 µm with an absorption feature at 10 µm, exhibiting the shape of a late Class 0 young stellar object (YSO). The radio continuum flux observed toward the nebula is consistent with the free-free emission from one or more massive YSOs (MYSOs) with spectral types in the range O9 to B0. This analysis demonstrates one technique the GLIMPSE team will use for revealing thousands of Galactic star formation regions.

DOI 10.1086/422815
Citations Scopus - 7
2003 Benjamin RA, Churchwell E, Babler BL, Bania TM, Clemens DP, Cohen M, et al., 'GLIMPSE. I. An SIRTF legacy project to map the inner galaxy', Publications of the Astronomical Society of the Pacific, 115 953-964 (2003)

The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), a Space Infrared Telescope Facility (SIRTF) Legacy Science Program, will be a fully sampled, confusion-limi... [more]

The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), a Space Infrared Telescope Facility (SIRTF) Legacy Science Program, will be a fully sampled, confusion-limited infrared survey of 2/3 of the inner Galactic disk with a pixel resolution of ~1¿.2 using the Infrared Array Camera at 3.6, 4.5, 5.8, and 8.0 µm. The survey will cover Galactic latitudes |b| = 1° and longitudes |l| = 10°-65° (both sides of the Galactic center). The survey area contains the outer ends of the Galactic bar, the Galactic molecular ring, and the inner spiral arms. The GLIMPSE team will process these data to produce a point-source catalog, a point-source data archive, and a set of mosaicked images. We summarize our observing strategy, give details of our data products, and summarize some of the principal science questions that will be addressed using GLIMPSE data. Up-to-date documentation, survey progress, and information on complementary data sets are available on the GLIMPSE Web site.

DOI 10.1086/376696
Citations Scopus - 729
2003 Bradford CM, Nikola T, Stacey GJ, Bolatto AD, Jackson JM, Savage ML, et al., 'CO (J = 7¿6) observations of NGC 253: Cosmic-ray-heated warm molecular gas', Astrophysical Journal, 586 891-901 (2003)

We report observations of the CO J = 7 ¿ 6 transition toward the starburst nucleus of NGC 253. This is the highest excitation CO measurement in this source to date and allows an ... [more]

We report observations of the CO J = 7 ¿ 6 transition toward the starburst nucleus of NGC 253. This is the highest excitation CO measurement in this source to date and allows an estimate of the molecular gas excitation conditions. Comparison of the CO line intensities with a large velocity gradient, escape probability model indicates that the bulk of the (2-5) × 10 7 M ¿ of molecular gas in the central 180 pc is highly excited. A model with T ~ 120 K, n H2 ~ 4.5 × 10 4 cm -3 , is consistent with the observed CO intensities, as well as the rotational H 2 lines observed with the Infrared Space Observatory. The inferred mass of warm, dense molecular gas is 10-30 times the atomic gas mass as traced through its [C II] and [O I] line emission. This large mass ratio is inconsistent with photodissociation region models in which the gas is heated by far-UV starlight. It is also not likely that the gas is heated by shocks in outflows or cloud-cloud collisions. We conclude that the best mechanism for heating the gas is cosmic rays, which provide a natural means of uniformly heating the full volume of molecular clouds. With the tremendous supernova rate in the nucleus of NGC 253, the cosmic-ray heating rate is at least ~800 times greater than that in the Galaxy, more than sufficient to match the cooling observed in the CO lines.

DOI 10.1086/367854
Citations Scopus - 80
2003 Kolpak MA, Jackson JM, Bania TM, Clemens DP, Dickey JM, 'Resolving the kinematic distance ambiguity toward Galactic H II regions', Astrophysical Journal, 582 756-769 (2003)

Kinematic distance determinations in the inner Galaxy are hampered by the near-far kinematic distance ambiguity. Here we resolve the ambiguity for 49 H II region complexes with kn... [more]

Kinematic distance determinations in the inner Galaxy are hampered by the near-far kinematic distance ambiguity. Here we resolve the ambiguity for 49 H II region complexes with known recombination-line velocities in the first Galactic quadrant. We measured the 21 cm H I absorption spectrum toward each source with the Very Large Array in the C array. The maximum velocity of H I absorption was used to discriminate between the near and far kinematic distances. The number ratio of far to near sources, ~3, can be entirely explained as a geometrical effect. The kinematic distances that we derive are compared with previous determinations for the same sources. Although our distance determinations are largely in agreement with previous measurements, there are 22 discrepancies that we discuss. Using our distance determinations, we create a face-on Galactic map of the H II region complexes and compare it with a kinematically derived profile of the distribution of CO-traced molecular hydrogen. The H II region complexes delineate the large-scale features seen in the molecular gas. The 5 kpc molecular ring and the Sagittarius spiral arm are clearly evident, and a few H II region complexes lie in the Perseus arm.

DOI 10.1086/344752
Citations Scopus - 78
2003 Kraemer KE, Jackson JM, Kassis M, Deutsch LK, Hora JL, Simon R, et al., 'Five star-forming cores in the galactic ring survey: A mid-infrared study', Astrophysical Journal, 588 918-930 (2003)

We have imaged five dense molecular cores, selected from the Galactic Ring Survey (GRS), in the mid-infrared with the MIRAC3 instrument. We obtained high spatial resolution (~1¿)... [more]

We have imaged five dense molecular cores, selected from the Galactic Ring Survey (GRS), in the mid-infrared with the MIRAC3 instrument. We obtained high spatial resolution (~1¿) images through narrowband filters at 12.5 and 20.6 µm. Four of the five cores show multiple compact sources, extended structure, or both. Lower resolution observations by the Infrared Space Observatory (ISO) and the Midcourse Space Experiment (MSX) suggest that the fifth core is also surrounded by extended emission on large scales (=2'). The extended mid-infrared structure is well-correlated with the radio continuum morphology in each of the five cores. This similarity suggests that the hot dust traced by the mid-infrared is located within the H II region, traced by the radio continuum, and not merely in a surrounding photodissociation region or molecular cloud, If a single exciting source is assumed for each core, estimates of the zero-age main-sequence spectral types based on the infrared luminosities are typically 1-2 spectral types earlier than those based on the radio free-free emission. However, allowing for multiple exciting sources and apportioning the far-infrared and radio fluxes to the component sources according to the mid-infrared flux distribution produces better agreement between the derived spectral types, with an average difference of less than half a spectral type.

DOI 10.1086/374264
Citations Scopus - 10
2003 Bolatto AD, Leroy A, Israel FP, Jackson JM, 'Unusual CO Line Ratios and Kinematics in the N83/N84 Region of the Small Magellanic Cloud', Astrophysical Journal, 595 167-178 (2003)

We present new CO (1 ¿ 0) and (2 ¿ 1) observations of the N83/ N84 molecular cloud complex in the southeast wing of the Small Magellanic Cloud (SMC). While the (2 ¿ 1)/(1 ¿ 0)... [more]

We present new CO (1 ¿ 0) and (2 ¿ 1) observations of the N83/ N84 molecular cloud complex in the southeast wing of the Small Magellanic Cloud (SMC). While the (2 ¿ 1)/(1 ¿ 0) integrated line brightness ratio (in temperature units) is uniformly 1.1 throughout most of the complex, we find two distinct regions with unusually high ratios, (2 ¿ 1)/(1 ¿ 0) ¿2. These regions are associated with the N84D nebula and with the inside of the 50 pc expanding molecular shell N83. This shell is spatially coincident with the NGC 456 stellar association and the HFPK2000-448 radio continuum/X-ray source, tentatively classified as a supernova remnant. We explore possible causes for the high ratios observed and conclude that the CO emission probably arises from an ensemble of small (R ~ 0.1 pc), warm (T g ~ 40 K) clumps. Analysis of the CO shell parameters suggests that it is wind driven and has an age of slightly more than 2 million years. We have also used this data set to determine the CO-to-H 2 conversion factor in the SMC, an especially interesting measurement because of the low metallicity of this source (¿1/9 solar). Surprisingly, after comparing the CO luminosities of clouds in N83/N84 with their virial masses, we find a CO-to-H 2 conversion factor X CO only 1.9 times larger than what we obtain when applying the same algorithm to solar metallicity clouds in the Milky Way and M33. This result fits into the emerging pattern that CO observations with high linear resolution suggest nearly Galactic values of X CO in a wide range of environments.

DOI 10.1086/377230
Citations Scopus - 35
2002 Bradford CM, Stacey GJ, Swain MR, Nikola T, Bolatto AD, Jackson JM, et al., 'SPIFI: A direct-detection imaging spectrometer for submillimeter wavelengths', Applied Optics, 41 2561-2574 (2002)

The South Pole Imaging Fabry¿Perot Interferometer (SPIFI) is the first instrument of its kind¿a direct-detection imaging spectrometer for astronomy in the submillimeter band. SP... [more]

The South Pole Imaging Fabry¿Perot Interferometer (SPIFI) is the first instrument of its kind¿a direct-detection imaging spectrometer for astronomy in the submillimeter band. SPIFI¿s focal plane is a square array of 25 silicon bolometers cooled to 60 mK; the spectrometer consists of two cryogenic scanning Fabry¿Perot interferometers in series with a 60-mK bandpass filter. The instrument operates in the short submillimeter windows (350 and 450 µm) available from the ground, with spectral resolving power selectable between 500 and 10,000. At present, SPIFI¿s sensitivity is within a factor of 1.5¿3 of the photon background limit, comparable with the best heterodyne spectrometers. The instrument¿s large bandwidth and mapping capability provide substantial advantages for specific astrophysical projects, including deep extragalactic observations. We present the motivation for and design of SPIFI and its operational characteristics on the telescope. © 2002 Optical Society of America.

DOI 10.1364/AO.41.002561
Citations Scopus - 26
2002 McQuinn KBW, Simon R, Law CJ, Jackson JM, Bania TM, Clemens DP, Heyer MH, 'A comparison of

We analyze 2 deg 2 of the Galactic plane surveyed in CS J = 2 ¿ 1 and 13 CO J = 1 ¿ 0 emission lines as a part of the Boston University-Five College Radio Astronomy Observator... [more]

We analyze 2 deg 2 of the Galactic plane surveyed in CS J = 2 ¿ 1 and 13 CO J = 1 ¿ 0 emission lines as a part of the Boston University-Five College Radio Astronomy Observatory Milky Way Galactic Ring Survey. Since the critical density of the CS molecule is large, strong CS emission originates only in dense molecular cloud cores. Yet, because high volume density regions traced by CS also tend to have large column densities, we find that 13 CO is just as useful as, and much more efficient than, CS for identifying potential dense, star-forming cores. Sixty-five percent of the star-forming sites in the survey region, selected using color criteria for embedded IRAS point sources, are detected as bright 13 CO clumps with emission above an integrated intensity of 15 K km s -1 (greater than 37 s). The fraction of those infrared point sources detected as bright CS clumps above 1 K km s -1 (greater than 3 s) is only 35%. The CS/ 13 CO intensity ratio can be used as a measure of gas excitation conditions. We compared the observed CS and 13 CO line intensities of the entire 2 deg 2 field as well as the average line ratios from two molecular clouds with very different physical properties. The average intensity ratio for GRSMC 45.46+0.05, a high volume density, star-forming molecular cloud, calculated with a high (26 K km s -1 ) 13 CO flux threshold, is T mb (CS)/T mb ( 13 CO) = 0.17 ± 0.06, with a peak value of ~0.5 toward two of the CS emission maxima. The ratio for the same cloud calculated using all 13 CO positions with flux above 3 s is 0.06 ± 0.01. For GRSMC 45.60+0.30, a low-density, quiescent molecular cloud, this ratio is even lower, 0.03 ± 0.01. The average line ratio for the entire 2 deg 2 field is 0.04 ± 0.01, similar to the value for the low-density cloud. Although the CS lines are brightest toward star-forming cores, ubiquitous, low-level CS emission dominates the emission in the survey region. This emission probably originates from subthermally excited, low-density gas.

DOI 10.1086/341538
Citations Scopus - 12
2002 Kolpak MA, Jackson JM, Bania TM, Dickey JM, 'The radial distribution of cold atomic hydrogen in the Galaxy', Astrophysical Journal, 578 868-876 (2002)

H I absorption spectra measured against bright extragalactic radio continuum sources offer a simple way to measure the optical depth of cold atomic gas as a function of Galactocen... [more]

H I absorption spectra measured against bright extragalactic radio continuum sources offer a simple way to measure the optical depth of cold atomic gas as a function of Galactocentric radius. We have observed the 21 cm H I line in absorption toward 54 bright, compact extragalactic radio continuum sources in the first Galactic quadrant with the VLA in C array. We have determined the average radial profile of 21 cm H I optical depth in the first quadrant using the Clemens rotation curve. There is a region of high average optical depth between Galactic radii of 4 and 8 kpc where the average optical depth, <t(R)>, exceeds 0.8. This region also contains most of the molecular gas in the Galaxy. Furthermore, the highest peak in <t(R)> occurs in the region of the 5 kpc molecular ring, the Galaxy's single most prominent molecular feature. Inward of 4 kpc, <t(R)> drops below 0.5, and beyond 8.5 kpc, <t(R)> remains below 0.4. The H I optical depth measurements were used to construct the 21 cm H I velocity-averaged absorption coefficient, <¿(R)>, as a function of Galactocentric radius. The average value of <¿(R)> exceeds 4.5 km s -1 kpc -1 between 4 and 8 kpc and drops sharply at smaller and larger radii. The highest peak in <¿(R)> rises above 15 km s -1 kpc -1 and again corresponds to the 5 kpc molecular ring. Two other large peaks correspond to the Sagittarius and Perseus spiral arms. We suggest that the high H I opacity in the inner Galaxy, particularly between 4 and 8 kpc, is due to the presence of cold atomic gas associated with molecular clouds.

DOI 10.1086/342659
Citations Scopus - 16
2002 Jackson JM, Bania TM, Simon R, Kolpak M, Clemens DP, Heyer M, 'H I self-absorption and the kinematic distance ambiguity: The case of the molecular cloud GRSMC 45.6+0.3', Astrophysical Journal, 566 (2002)

We report on a three-dimensional (l, b, V LSR ) physical association of H I self-absorption with 13 CO emission toward the Galactic molecular cloud GRSMC 45.6+0.3. Photochemical ... [more]

We report on a three-dimensional (l, b, V LSR ) physical association of H I self-absorption with 13 CO emission toward the Galactic molecular cloud GRSMC 45.6+0.3. Photochemical models indicate that a significant column density of cool (T = 25 K) H I exists in the skins (A v = 2) of typical Galactic molecular clouds. The H I self-absorption can thus be produced by the cool cloud skin absorbing the warm (T SPIN ~ 100 K) background H I emission that is ubiquitously distributed throughout the Galaxy. Liszt, Burton, and Bania have suggested that because cool H I in a molecular cloud will absorb warmer H I background line emission, H I self-absorption can be used to resolve the kinematic distance ambiguity for molecular clouds in the inner Galaxy. Clouds at the near distance should show H I self-absorption, whereas clouds at the far distance will not since there is no background to absorb. The near kinematic distance of 1.8 kpc for the GRSMC 45.6+0.3 molecular cloud is independently confirmed by its morphological correlation with foreground optical extinction.

DOI 10.1086/339715
Citations Scopus - 43
2001 Stark AA, Bally J, Balm SP, Bania TM, Bolatto AD, Chamberlin RA, et al., 'The antarctic submillimeter telescope and remote observatory (AST/RO)', Publications of the Astronomical Society of the Pacific, 113 567-585 (2001)

The Antarctic Submillimeter Telescope and Remote Observatory, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 µm, was installed ... [more]

The Antarctic Submillimeter Telescope and Remote Observatory, a 1.7 m diameter telescope for astronomy and aeronomy studies at wavelengths between 200 and 2000 µm, was installed at the South Pole during the 1994-1995 austral summer. The telescope operates continuously through the austral winter and is being used primarily for spectroscopic studies of neutral atomic carbon and carbon monoxide in the interstellar medium of the Milky Way and the Magellanic Clouds. The South Pole environment is unique among observatory sites for unusually low wind speeds, low absolute humidity, and the consistent clarity of the submillimeter sky. Especially significant are the exceptionally low values of sky noise found at this site, a result of the small water vapor content of the atmosphere. Four heterodyne receivers, an array receiver, three acousto-optical spectrometers, and an array spectrometer are currently installed. A Fabry-Perot spectrometer using a bolometric array and a terahertz receiver are in development. Telescope pointing, focus, and calibration methods as well as the unique working environment and logistical requirements of the South Pole are described.

DOI 10.1086/320281
Citations Scopus - 20
2001 Kraemer KE, Jackson JM, Deutsch LK, Kassis M, Hora JL, Fazio GG, et al., 'Dust characteristics of massive star-forming sites in the mid-infrared', Astrophysical Journal, 561 282-298 (2001)

Four massive star-forming regions were imaged in the mid-infrared with the MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtained high spatial resolution (~1¿) images... [more]

Four massive star-forming regions were imaged in the mid-infrared with the MIRAC3 instrument: W51 IRS 2, Mon R2, DR 21, and S140. We obtained high spatial resolution (~1¿) images at several wavelengths from 7.8 to 13.2 µm with the circular variable filter, as well as narrow-band continuum images at 12.5 and 20.6 µm toward each region. In each massive star-forming region, one or more sources show deep silicate absorption. For at least two of the massive star-forming regions, W51 IRS 2 and Mon R2, the absorbing material is highly localized and may be circumstellar material in disks or shells. The silicate absorption occurs at least as often around massive young stars as around young stars of lower mass (which are more often observed). The estimated optical depths of the silicate features are consistent with those predicted by radiative transfer models toward ultracompact H II regions, but substantially higher than observed toward T Tauri stars and other low-mass young stellar objects. There is no consistent correspondence between silicate absorption and either the dust color temperature or the 12.5 µm opacity. In W51 IRS 2, the two previously known mid-infrared sources have been resolved into at least six subsources. Infrared counterparts are newly reported for two radio-continuum sources in S140. Also, new mid-infrared sources have been detected in both W51 IRS 2 and S140. We suggest that the infrared source in the southwest of DR 21 may not be self-luminous, but may instead be heated by the three nearby radio continuum sources. The gas density in the ring at Mon R2 supports the blister scenario for the IRS 1 H II region.

DOI 10.1086/323219
Citations Scopus - 20
2001 Simon R, Jackson JM, Clemens DP, Bania TM, Heyer MH, 'The structure of four molecular cloud complexes in the BU-FCRAO milky way galactic ring survey', Astrophysical Journal, 551 747-763 (2001)

We present a study of the structure of four molecular clouds from the Milky Way Galactic Ring Survey (GRS), a Boston University and Five College Radio Astronomy Observatory collab... [more]

We present a study of the structure of four molecular clouds from the Milky Way Galactic Ring Survey (GRS), a Boston University and Five College Radio Astronomy Observatory collaboration. The GRS is a new high-resolution survey in the 13 CO J = 1 ¿ 0 spectral line of the inner Galaxy and the 5 kpc ring, the Milky Way's dominant star-forming structure. Because of the smaller line widths of 13 CO compared to 12 CO, we can avoid velocity crowding and establish accurate kinematic distances to the clouds. The kinematic distance ambiguity in the first Galactic quadrant is resolved using self-absorption in complementary high-resolution atomic hydrogen data. The four clouds are selected to span a large range of star formation activity, from the quiescent cloud GRSMC 45.60 + 0.30, which shows no signs of high-mass star formation, to W49, the most luminous star-forming region in the Galaxy. We use a three-dimensional Gaussian clump decomposition to identify clumps in the clouds and to investigate their properties. Each cloud has the same clump mass spectrum, dN/dM ¿ M -1.8 , independent of star formation activity. We do not find significant differences in the slopes of the relations of density, line width, and clump mass as a function of clump size among the clouds. The size-density and size-line width relations show considerable scatter. Compared to the conventional Larson scaling laws, we find systematically natter slopes for the size-density and size-line width relations and a higher power-law index for the size-mass relation. In particular, the clump line widths for the most quiescent cloud GRSMC 45.60 + 0.30 are independent of clump size. While the clouds as a whole are gravitationally bound, most of the clumps are not; only a small fraction of the total number of clumps is self-gravitating. The active star-forming clouds have a higher fraction of gravitationally bound clumps and a higher mean cloud volume density than the more quiescent clouds. The gravitationally unbound clumps are possibly confined by the weight of the self-gravitating complex. The pressures needed to bind these clumps are largest for the active star-forming clouds, which have a much higher weight than the quiescent clouds. Alternatively, a high number of the gravitationally unbound clumps may be transien t.

DOI 10.1086/320230
Citations Scopus - 141
2001 Ojha R, Stark AA, Hsieh HH, Lane AP, Chamberlin RA, Bania TM, et al., 'AST/RO Observations of Atomic Carbon near the Galactic Center', Astrophysical Journal, 548 253-253 (2001)
2000 Bolatto AD, Jackson JM, Wilson CD, Moriarty-Schieven G, 'Submillimeter observations of IC 10: The dust properties and neutral carbon content of a low-metallicity starburst', Astrophysical Journal, 532 909-921 (2000)

We present submillimeter observations of the Local Group, metal-poor, irregular dwarf galaxy IC 10, directly relevant to the interaction between interstellar medium (ISM) and star... [more]

We present submillimeter observations of the Local Group, metal-poor, irregular dwarf galaxy IC 10, directly relevant to the interaction between interstellar medium (ISM) and star formation activity in primeval galaxies. Using the James Clerk Maxwell Telescope, we have observed the fine-structure neutral carbon transition 3 P 1 -» 3 P 0 at 492 GHz and the rotational J = 1 ¿ 2 transition of 12 CO and 13 CO in the most massive giant molecular cloud complex in this galaxy, IC 10-SE. We find that, although the I [C n] /I co ratio for this object is a factor of 4 larger than the typical Milky Way value, its [C I] to CO intensity ratio I [CI] /I CO -18 ± 2 (in units of ergs s -1 cm -2 s -1 ) is similar (only about 50% larger) to that of the Milky Way. Modelling of the behavior of the [C II]/CO and [C I] /CO intensity ratios with metallicity indicates that, if C + and C° are chiefly produced by UV photodissociation in the photodissociation region, both ratios should increase sharply with decreasing metallicity (and consequently diminished UV shielding). These data then suggest a different origin for an important fraction of C° in these clouds, unrelated to photodissociation. We have also mapped the 850 µm continuum in this region using the Submillimetre Common User Bolometer Array. Employing these data in conjunction with KAO and IRAM measurements we find that the 100-1300 µm continuum emission corresponds to a graybody with an extremely low emissivity exponent, ß¿ 0.5. We conclude that this low exponent is most likely due to the destruction of small dust grains, brought about by the increased penetration of UV radiation in the low-metallicity ISM. If a low emissivity exponent in the submillimeter is a general property of metal-poor systems, then the interpretation of millimeter and submillimeter surveys of high-z galaxies should be revised.

Citations Scopus - 24
2000 Smith N, Jackson JM, Kraemer KE, Deutsch LK, Bolatto A, Hora JL, et al., 'Thermal infrared imaging of ultracompact H II regions in W49A', Astrophysical Journal, 540 316-331 (2000)

Several compact radio continuum sources in W49A show detectable 8-20 µm emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources clo... [more]

Several compact radio continuum sources in W49A show detectable 8-20 µm emission in MIRAC2 images obtained at the IRTF. In general, the infrared morphologies of these sources closely resemble the radio continuum emission. Spectral energy distributions indicate an infrared continuum excess above the level expected from free-free emission, consistent with thermal emission from dust grains heated to a few hundred K. The bright radio continuum sources concentrated at the western end of the ring of ultracompact H II regions are not detected in the mid-infrared, while those at other positions in the ring are detected. This could be due to a localized region of high extinction along the line of sight. In addition, there are a few new infrared sources with no radio continuum counterparts. Finally, several infrared sources show strong 12.8 µm [Ne II] emission, yielding neon abundances that are typically a few percent of the cosmic abundance of neon but are high considering the e xpected Ne + + /Ne + ratios for the range of spectral types of the ionizing sources. We conclude that the [Ne II] emission must come from shells around the ultracompact H II regions, where the neon is able to survive as Ne + rather than Ne + + because the radiation field has been softened by absorption of hard UV photons within the H II regions.

Citations Scopus - 9
2000 Burton MG, Ashley MCB, Marks RD, Schinckel AE, Storey JWV, Fowler A, et al., 'High-resolution imaging of photodissociation regions in NGC 6334', Astrophysical Journal, 542 359-366 (2000)

We have used the SPIREX telescope to conduct a wide-field thermal infrared imaging study of the star formation complex NGC 6334 in the southern Galactic plane. We imaged a 30&apos... [more]

We have used the SPIREX telescope to conduct a wide-field thermal infrared imaging study of the star formation complex NGC 6334 in the southern Galactic plane. We imaged a 30' region along the main star-forming ridge of NGC 6334 with 0¿.6 pixel scale through broadband filters for L (3.5 µm) and M (4.8 µm) and through narrowband filters for the H 2 ¿ = 1-0 Q-branch (2.42 µm), polycyclic aromatic hydrocarbon (PAH) (3.3 µm), and Bra (4.05 µm) lines. The images reveal the spectacular, complex structure of the photodissociation regions (PDRs) that pervade the region, with enhanced line emission around each of the seven sites of massive star formation along the ridge. Bubbles and loops of PAH emission, typically 1-1.5 pc across, have been carved out of the parent molecular cloud by the intense UV radiation from the massive stars and surround H II regions (seen in Bra) typically 0.2-0.3 pc across. The PAH emission regions coincide with both [C II] 158 µm line emission, indicating that the PAHs are excited in PDR gas, and extensive H 2 emission, which therefore must be fluorescent. However, the textures of the emission regions in PAH and H 2 are different. This is attributable to variations in the physical environment in which the gas is excited. Several compact reddened objects are observed; these are likely to be massive protostars.

Citations Scopus - 32
2000 Bolatto AD, Jackson JM, Israel FP, Zhang X, Kim S, 'Carbon in the N159/N160 complex of the Large Magellanic Cloud', Astrophysical Journal, 545 234-250 (2000)

We present a study of carbon in N159/N160, an H II region complex in the low-metallicity Large Magellanic Cloud. We have mapped this region, which comprises four distinct molecula... [more]

We present a study of carbon in N159/N160, an H II region complex in the low-metallicity Large Magellanic Cloud. We have mapped this region, which comprises four distinct molecular clouds spanning a wide range of star formation activity, in four transitions: 13 CO (J = 1 ¿ 0), 12 CO (J = 2 ¿ 1) and (J = 4 ¿ 3), and [C I] ( 3 P 1 ¿ 3 P 0 ). Combining these data with existing [C II] ( 2 P 312 ¿ 2 P 1/2 ) observations provides a complete picture of the predominant forms of carbon in the gas phase of the ISM. The new CO (J = 2 ¿ 1) data show that the complex is immersed in an envelope of extended, low-level emission, undetected by previous (J = 1 ¿ 0) mapping efforts. The 12 CO (J = 2 ¿ 1)/(J = 1 ¿ 0) ratio in this envelope is ¿ 3, a value consistent with optically thin CO emission. The envelope is also relatively bright in [C I] and [C II] , and calculations show that it is mostly photodissociated: it appears to be translucent (A V < 1). Neutral carbon emission in the complex unexpectedly peaks at the quiescent southern cloud (N159S). In the northern portion of the map (the N160 nebula), the H II regions prominent in [C II] correspond to holes in the [C I] distribution. Overall we find that, while the I [CII] /I CO ratio is enhanced with respect to similar complexes in the Milky Way, the I [CII] /I CO ratio appears to be similar or reduced.

Citations Scopus - 47
2000 Kraemer KE, Jackson JM, Lane AP, Paglione TAD, 'Properties of the photodissociated gas in NGC 6334', Astrophysical Journal, 542 946-956 (2000)

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-PÃ... [more]

To investigate the properties of photodissociated gas in strong radiation fields, the massive star formation region NGC 6334 has been mapped with the Far-Infrared Imaging Fabry-Pérot Interferometer (FIFI) instrument in the far-infrared fine-structure transitions [C II] 158 µm, [O I] 146 µm, and [O I] 63 µm. Bright [C II] 158 µm emission is found throughout the cloud, whereas the [O I] 146 µm emission is associated only with the star-forming ridge. Comparison of the relative intensities of the lines with single-component photodissociation region models suggests densities of n H ~ 10 4 cm -3 . The models imply unphysically large radiation fields for three sources, particularly for NGC 6334A, which is probably caused by self-absorption in the [O I] 63 µm line. An alternative method for estimating physical conditions, based on the correlation between [C II] 158 µm and CO line intensities, is explored. This method implies hydrogen column densities of N H ¿ 10 22-23 cm -2 , which agree well with those from molecular excitation models. The relative distribution of the [C II] 158 µm and [O I] 146 µm emission supports clumpy photodissociation region models that suggest the [C II] 158 µm emission arises from interclump gas and thus should be more extended than the [O I] 146 µm emission that arises from the denser clumps. The spatial coincidence of [C II] 158 µm emission peaks with minima in the molecular gas emission indicates that at least some of the molecular holes contain photodissociated gas. The relative pressures of the ionized, photodissociated, and molecular gas are compared: the photodissociated and molecular gas are in approximate pressure equilibrium, but the ionized gas is overpressurized by at least an order of magnitude at the continuum sources.

Citations Scopus - 5
2000 Bolatto AD, Jackson JM, Kraemer KE, Zhang X, 'First detection of submillimeter [C I] emission in the Small Magellanic Cloud', Astrophysical Journal, 541 (2000)

We report the first detection of [C I] ( 3 P 1 ¿ 3 P 0 ) emission at 609 µm in a region of the Small Magellanic Cloud (N27). Environments poor in heavy elements and dust such ... [more]

We report the first detection of [C I] ( 3 P 1 ¿ 3 P 0 ) emission at 609 µm in a region of the Small Magellanic Cloud (N27). Environments poor in heavy elements and dust such as the SMC are thought to be dominated by photodissociation regions (PDRs). This is the lowest metallicity source where submillimeter neutral carbon emission has been detected. Studying the I [C I] /I CO ratio in several sources spanning more than an order of magnitude in metallicity Z, we find that the I [C I] /I CO ratio increases for decreasing Z. The existence of such a trend points to a photodissociation origin for most of the neutral carbon in molecular clouds, in agreement with standard PDR models. We also report Infrared Space Observatory far-infrared (FIR) spectroscopic observations of N27 and use them to derive its physical properties. Comparison between the density and radiation field revealed by FIR diagnostics (n ~ 300-1000 cm -3 , ¿ uv ~ 30¿ 0 -100¿ 0 ) and those derived from millimeter and submillimeter data (n ~ 10 5 cm -3 , ¿ uv ¿ 30¿ 0 ) suggests that the FIR lines originate in more diffuse gas and are perhaps dominated by the interclump medium. Regardless of the cause, analysis of the FIR and millimeter-submillimeter data produces a discrepancy of 2 orders of magnitude for the density of this source.

Citations Scopus - 8
1999 Bolatto AD, Jackson JM, Ingalls JG, 'A semianalytical model for the observational properties of the dominant carbon species at different metallicities', Astrophysical Journal, 513 275-286 (1999)

We present a simple, semianalytical approach to compute the line emission of C + , C 0 , and CO in photodissociation regions of varying metallicity that depends on very few parame... [more]

We present a simple, semianalytical approach to compute the line emission of C + , C 0 , and CO in photodissociation regions of varying metallicity that depends on very few parameters and naturally incorporates the dumpiness of the interstellar medium into the calculations. We use it to study the dependence of the I [C II] /I CO and I [C I] /I CO line ratios on the metallicity. We show how to modify it to include the effects of density and radiation field, and we have compared it with observational data. We find that the model explains the observed trend of enhanced [C II]/CO line ratio with decreasing metallicity as the natural result of the augmented fraction of photodissociated gas in a clump. We show that enhanced [C II] /CO ratios can be produced by lowering the upper limit of the clump size distribution, as may happen in 30 Doradus. We also find that the available data favor a [C I]/CO intensity ratio essentially independent of metallicity, albeit the paucity of observations does not exclude other possibilities. This is difficult to understand if most of the C 0 is produced by UV photons as is the case with C + . Finally, we study the prediction of the model for the trend of the X co factor with metallicity. Comparison with previous observational studies yields good agreement.

Citations Scopus - 54
1999 Huang M, Bania TM, Bolatto A, Chamberlin RA, Ingalls JG, Jackson JM, et al., 'Atomic carbon observations of southern hemisphere H II regions', Astrophysical Journal, 517 282-291 (1999)

We report observations of atomic carbon (C I [ 3 P 1 ¿ 3 P 0 ]) for a sample of 49 southern hemisphere H II regions using the Antarctic Submillimeter Telescope and Remote Obser... [more]

We report observations of atomic carbon (C I [ 3 P 1 ¿ 3 P 0 ]) for a sample of 49 southern hemisphere H II regions using the Antarctic Submillimeter Telescope and Remote Observatory. The sources are compact and isolated members of the Wilson et al. H109a radio recombination line (RRL) catalog. The fourth Galactic quadrant is well covered by the sample. Atomic carbon emission is detected toward all of the regions, with multiple C I emission components found toward most sources. The RRL velocity is used to identify the C I emission associated with the H II region. We measure the mean velocity difference between the C I and RRL emission to be 0.8 ± 2.8 km s -1 . Within the measurement errors this is exact agreement in velocity; we conclude that all H II regions have associated C I emission. The mean C I line temperature of these components is 2.4 ± 1.8 K, compared with 0.7 ± 0.7 K for the C I emission components not associated with the H II region. This suggests that C I intensity is dominated by local heating. The FWHM line width of C I gas associated with H II regions also is marginally greater than that found for unassociated gas (6.7 ± 3.0, compared with 4.8 ± 2.4 km s -1 ).

Citations Scopus - 14
1999 Jackson JM, Kraemer KE, 'Photodissociation regions and H II regions in NGC 6334', Astrophysical Journal, 512 260-270 (1999)

Using the VLA at 8.485 GHz, we have imaged the southern portion of the star-forming ridge of molecular gas in NGC 6334. The diffuse radio source G351.20+0.70, discovered by Moran ... [more]

Using the VLA at 8.485 GHz, we have imaged the southern portion of the star-forming ridge of molecular gas in NGC 6334. The diffuse radio source G351.20+0.70, discovered by Moran et al. (1990), is now resolved into a roughly spherical shell of radius ~1' (0.5 pc). The distribution of molecular gas (traced by CO emission), of photodissociated gas (traced by [C II] 158 µm emission), and of ionized gas (traced by radio continuum emission), is precisely that expected for a photodissociation region - the ionized gas lies on the interior of the shell, the photodissociated gas just outside the ionized gas, and the molecular gas just outside the photodissociated gas. We also detected faint radio counterparts to the strong infrared sources NGC 6334 IV IRS 20 and NGC 6334 V. If these objects are zero-age main-sequence stars, they produce far less radio free-free emission than would be expected for the observed infrared flux. Some possible explanations for this discrepancy are the following: (1) the radio free-free emission is optically thick, (2) the stellar ionizing radiation is obscured by dust, (3) the objects are not single OB stars but very compact clusters of later type stars, or (4) the objects are protostars. For both NGC 6334 V and NGC 6334 IV IRS 20, the radio spectrum for the unresolved sources is inconsistent with optically thick free-free emission or dust obscuration from a homogeneous H II region. The radio spectral index for NGC 6334 IV IRS 20 is consistent with the value of 0.6 expected for an optically thick H II region for a star undergoing mass loss, but that of NGC 6334 V is not. Because the IR sources in NGC 6334 V are very compact (¿0.02 pc), the stellar volume densities for a cluster of later-type stars would be unreasonably large. The objects in NGC 6334 V are probably protostars.

Citations Scopus - 25
1999 Kraemer KE, Deutsch LK, Jackson JM, Hora JL, Fazio GG, Hoffmann WF, Dayal A, 'The mid-infrared properties of three star-forming sites in NGC 6334', Astrophysical Journal, 516 817-833 (1999)

To investigate their dust properties, we have imaged three sites of massive star formation in the giant H II region/star-forming cloud NGC 6334 with the MIRAC2 instrument. We obta... [more]

To investigate their dust properties, we have imaged three sites of massive star formation in the giant H II region/star-forming cloud NGC 6334 with the MIRAC2 instrument. We obtained high-resolution (1¿) continuum images at 12.5 and 20.6 µm toward each region, which were compared with observations of the radio and near-infrared (near-IR) continuum emission. Both compact sources and extended emission were found at all three star-forming sites. The detected sources span a wide range of evolutionary states in this highly complex star-forming cloud. The infrared sources near NGC 6334 I were resolved into at least four subsources. One such source is substantially colder, denser, and more optically thick than the other mid-IR sources in the region and may be at the earliest stages of stellar formation. Another may be a torus or disk of dust and gas surrounding an embedded B star. NGC 6334 I was also imaged at additional wavelengths (8.8, 9.8, and 11.7 µm) to search for silicate absorption. Only at the H II region is there a deep silicate absorption feature from foreground dust. Toward the NGC 6334 IV, warm dust is associated with both the inner portions of the massive molecular torus or disk and with the bipolar continuum lobes. A compact mid-IR source, associated with the near-IR and radio source [HHS87] 1RS 20, is cooler and more optically thick than the dust emission associated with the H II region. Toward NGC 6334 V, four embedded sources were found, including one previously unidentified object. This newly identified compact object, associated with a dust temperature peak and a radio source, is probably in a more advanced stage of star formation than the other compact mid-IR sources in NGC 6334.

Citations Scopus - 32
1999 Kraemer KE, Jackson JM, 'Molecular gas in the NGC 6334 star formation region', Astrophysical Journal, Supplement Series, 124 439-463 (1999)

We present millimeter- and centimeter-wave spectroscopic observations of the southern massive star formation region NGC 6334. The cloud has been mapped in several transitions of C... [more]

We present millimeter- and centimeter-wave spectroscopic observations of the southern massive star formation region NGC 6334. The cloud has been mapped in several transitions of CO, 13 CO, CS, and NH 3 . The molecular gas shows a complex structure of filaments, in which the massive star formation occurs, and bubbles, some of which contain photodissociated gas. There is an anticorrelation between the presence of dense gas and the 6 cm radio flux: the hottest stars, with the hardest FUV radiation, have dispersed the dense gas from which they formed, whereas the cooler stars have not yet been able to do so. There is a velocity gradient along the star-forming ridge such that the radial velocity peaks in the center of the ridge. Several blueshifted emission features were discovered, one of which was identified with the "3-kpc" arm of the Galaxy. Excitation model calculations were used to determine the physical conditions of the molecular gas in NGC 6334. The average kinetic temperature, hydrogen volume, and column densities at the continuum sources are as follows: T¯ k = 56 ± 11 K, log n¯ H2 (cm -3 ) = 3.5 ± 0.3, and N¯ H2 (10 22 cm -2 ) = 7 ± 4, respectively. The properties of the molecular gas are compared to those in other massive star-forming clouds to determine that NGC 6334 is representative of massive star-forming regions in the Galaxy and can therefore be used to test the predictions of the theoretical models of photodissociation regions. The properties of the individual sites of star formation in the cloud are also discussed.

DOI 10.1086/313260
Citations Scopus - 31
1998 Kraemer KE, Jackson JM, Lane AP, 'Erratum: "[O I] 63 Micron Absorption in NGC 6334" (ApJ, 503, 785 [1998])', Astrophysical Journal, 509 931-931 (1998)
1998 Kraemer KE, Jackson JM, Lane AP, '[O I] 63 micron absorption in NGC 6334', Astrophysical Journal, 503 785-791 (1998)

The [O I] 63 µm transition has been imaged around five far-infrared (FIR) and radio continuum sources in the southern massive star formation region NGC 6334. The [O I] 63 µm li... [more]

The [O I] 63 µm transition has been imaged around five far-infrared (FIR) and radio continuum sources in the southern massive star formation region NGC 6334. The [O I] 63 µm line is found in absorption toward the FIR continuum source NGC 6334V. This is only the second case in which the [O I] 63 µm line has been seen in absorption against a continuum source. From the depth of the absorption line, the minimum column density of oxygen is calculated to be N(O°)¿ 5 × 10 18 cm -2 . This amount of oxygen is consistent with [O I] 63 µm absorption due to atomic gas in the foreground molecular cloud. The [O I] 63 µm line is found in emission toward the other four sources observed: NGC 6334, sources A, C, D, and E. Single-component photodissociation region (PDR) models suggest densities of n ~ 10 4 cm -3 for these sources, based on previously observed [O I] 145 µm and [C II] 158 µm intensities. However, unphysically large far-ultraviolet (FUV) fields are implied for three of the sources, particularly for NGC 6334A. Neither one- nor two-component photodissociation region models can explain the anomalously low [O I] 63 µm intensity toward NGC 6334A nor the absorption toward NGC 6334V. We suggest that self-absorption of the [O I] 63 µm line, such as has been suggested toward DR 21, is suppressing the observed [O I] 63 µm intensity. This underestimate leads to an overestimate of the derived FUV field strengths throughout NGC 6334. The discovery of several more star-forming sites in which the [O I] 63 µm is in absorption or is self-absorbed implies that this line is not always a reliable PDR diagnostic because the PDR models do not treat the radiative transfer through the molecular cloud. © 1998. The American Astronomical Society. All rights reserved.

DOI 10.1086/306002
Citations Scopus - 21
1998 Paglione TAD, Jackson JM, Bolatto AD, Heyer MH, 'Interpreting the HCN/CO intensity ratio in the galactic center', Astrophysical Journal, 493 680-693 (1998)

We study the dense molecular gas in the Galactic center using maps of HCN and CO J = 1 ¿ 0 emission from the central 630 pc of the Milky Way, and images of HCN J = 3 ¿ 2 emissio... [more]

We study the dense molecular gas in the Galactic center using maps of HCN and CO J = 1 ¿ 0 emission from the central 630 pc of the Milky Way, and images of HCN J = 3 ¿ 2 emission, which requires high densities for excitation (n H2 ¿ 10 6 cm -3 ), from Sgr A and Sgr B. The ratio of integrated HCN and CO J = 1 ¿ 0 intensities is a sensitive measure of molecular gas pressure, and the ratio of integrated HCN J = 3 ¿ 2 and J = 1 ¿ 0 intensities uncovers density enhancements in the maps. However, the HCN/CO ratio is difficult to model without knowing the relative HCN and CO abundances. Furthermore, because of the different filling factors of HCN and CO emission, models that use homogeneous clouds may not be accurate for analyzing the HCN/CO ratio. Most of the mass traced by HCN and CO in the Galactic center is at high densities (n H2 ~ 10 4 cm -3 ), roughly an order of magnitude higher than cloud densities in the Galactic disk. Most of the dense gas traced by HCN J = 3 ¿ 2 emission is coincident with star-forming regions and cloud interaction zones, and not necessarily with emission peaks. We smoothed the HCN and CO maps to the typical spatial resolution of extragalactic observations, and repeated the analysis. The single large-scale measurement was sensitive to the mass-weighted average properties of the map. Therefore, if we can extrapolate this result to other spirals, studies such as this are sensitive to the average gas properties in galactic nuclei, despite poor spatial resolution. © 1998. The American Astronomical Society. All rights reserved.

DOI 10.1086/305136
Citations Scopus - 19
1997 Stark AA, Boltto AD, Chamberlin RA, Lane AP, Bania TM, Jackson JM, Lo KY, 'First detection of 492 GHz [C I] emission from the large magellanic cloud', Astrophysical Journal, 480 (1997)

The 3 P 1 ¿ 3 P 0 fine-structure transition of neutral atomic carbon [C I] was observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) toward two ... [more]

The 3 P 1 ¿ 3 P 0 fine-structure transition of neutral atomic carbon [C I] was observed with the Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) toward two star-forming regions in the Large Magellanic Cloud: N159 and 30 Doradus. The [C I] line is weak in the vicinity of 30 Dor, a region with a uniquely hard and intense UV field. The I[C I]I CO ratio in N159 is enhanced by a factor ¿2 compared to the Milky Way Galaxy, a result attributable to the lower metallicity of the Large Magellanic Cloud. © 1997. The American Astronomical Society. All rights reserved.

Citations Scopus - 22
1997 Kraemer KE, Jackson JM, Paglione TAD, Bolatto AD, 'A 2000 M

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH 3 . The molec... [more]

We present millimeter and centimeter wave spectroscopic observations of the H II region NGC 6334A. We have mapped the source in several transitions of CO, CS, and NH 3 . The molecular emission shows a distinct flattened structure in the east-west direction. This structure is probably a thick molecular disk or torus (2.2 × 0.9 pc) responsible for the bipolarity of the near-infrared (NIR) and radio continuum emission which extends in two "lobes" to the north and south of the shell-like H II region. The molecular disk is rotating from west to east (¿ ¿ 2.4 km s -1 pc -1 ) about an axis approximately parallel to the radio and NIR emission lobes. By assuming virial equilibrium, we find that the molecular disk contains ~2000 M ¿ . Single-component gas excitation model calculations show that the molecular gas in the disk is warmer and denser (T k ¿60 K, n ¿ 3000 cm -3 ) than the gas to the north and south (T k ¿ 50 K, n ¿ 400 cm -3 ). High resolution (~5¿) NH 3 (3, 3) images of NGC 6334A reveal several small (~0.1 pc) clumps, one of which lies southwest of the radio continuum shell, and is spatially coincident with a near-infrared source, IRS 20. A second NH 3 clump is coincident with an H 2 O maser and the center of a molecular outflow. The dense gas tracers, CS J = 5 ¿ 4 and 7 ¿ 6, peak near IRS 20 and the H 2 O maser, not at NGC 6334A. IRS 20 has a substantial far-infrared (FIR) luminosity L FIR ~ 10 5 L ¿ , which indicates the presence of an O 7.5 star but has no detected radio continuum (F 6cm < 0.02 Jy). The combination of dense gas, a large FIR luminosity and a lack of radio continuum can best be explained if IRS 20 is a protostar. A third clump of NH 3 emission lies to the west of IRS 20 but is not associated with any other molecular or continuum features. The star formation activity in the region has moved west of NGC 6334A to IRS 20 and the H 2 O maser position. We suggest that NGC 6334A, IRS 20, and the H 2 O maser spot are part of a "protocluster" of stars which is condensing from the massive molecular disk. The similarity between the structure around NGC 6334A and other large (r ~ 1 pc), massive (M ~ 10 3 M ¿ ), rotating disks (K3-50A and G10.6-0.4) suggests that this may be a common mechanism by which open clusters form. © 1997. The American Astronomical Society. All rights reserved.

DOI 10.1086/303827
Citations Scopus - 18
1997 Ingalls JG, Chamberlin RA, Bania TM, Jackson JM, Lane AP, Stark AA, 'Atomic carbon in southern hemisphere high-latitude clouds', Astrophysical Journal, 479 296-302 (1997)

We report the detection of atomic carbon in a sample of eight southern hemisphere high Galactic latitude molecular clouds, using the Antarctic Submillimeter Telescope and Remote O... [more]

We report the detection of atomic carbon in a sample of eight southern hemisphere high Galactic latitude molecular clouds, using the Antarctic Submillimeter Telescope and Remote Observatory. The 492 GHz ( 3 P 1 ¿ 3 P 0 ) transition of [C I] was detected in all of the clouds observed. The C/CO column density ratio ranges from 0.4 to 2.5 and is similar to the values previously measured in high-latitude clouds MBM 12 and HD 210121. For all 10 high-latitude clouds observed in [C I] , C/CO averages ~ 1.2 and decreases with increasing total gas column density N H , as predicted by translucent cloud models. Quantitative comparison with chemical models of homogeneous clouds is unsatisfactory, however, and we conclude that the clumpy structure of clouds must be taken into account in order to interpret the data properly. © 1997. The American Astronomical Society. All rights reserved.

DOI 10.1086/303876
Citations Scopus - 27
1997 Paglione TAD, Jackson JM, Ishizuki S, 'The average properties of the dense molecular gas in galaxies', Astrophysical Journal, 484 656-663 (1997)

We have observed the HCN J = 3 ¿ 2 and J = 1 ¿ 0 emission from several nearby starburst and normal galaxies. These lines have large critical densities (n H2 &gt; 10 6 cm -3 ... [more]

We have observed the HCN J = 3 ¿ 2 and J = 1 ¿ 0 emission from several nearby starburst and normal galaxies. These lines have large critical densities (n H2 > 10 6 cm -3 ) and excitation energies Thus, they probe the warm and dense molecular gas where massive stars typically form. The average cloud densities of these galaxies, as estimated from the ratio of the HCN J = 3 ¿ 2 and 1 ¿ 0 integrated intensities, is correlated with their star formation efficiency. Therefore, the average densities of the molecular clouds in starburst nuclei are higher than those of more quiescent galaxies. Further, the starburst galaxies NGC 253 and M82 have a much higher fraction of molecular mass at high density (n H2 > 10 4 cm -3 ) than the normal galaxies IC 342 and the Milky Way. These results imply that the clouds in starburst nuclei form stars more efficiently than those in normal galaxies. © 1997. The American Astronomical Society. AH rights reserved.

DOI 10.1086/304356
Citations Scopus - 37
1996 Jackson JM, 'HCN and CO in the central 630 parsecs of the galaxy', Astrophysical Journal, 456 (1996)

Using the QUARRY focal plane array at the FCRAO 14 m, we have mapped the central 630 × 75 pc (l × b = 4°.3 × 0×.5) of the Galaxy at ~ 50¿ (2 pc) resolution in both CO and HC... [more]

Using the QUARRY focal plane array at the FCRAO 14 m, we have mapped the central 630 × 75 pc (l × b = 4°.3 × 0×.5) of the Galaxy at ~ 50¿ (2 pc) resolution in both CO and HCNJ = 1 ¿ 0 emission. Since HCN emission traces gas with densities ¿100 times larger than that traced by CO, the HCN luminosity measures the mass of dense (n ¿ 10 5 cm -3 ) molecular gas, and the HCN/CO intensity ratio measures gas density. The densest molecular gas is found in the molecular clouds associated with the l = 1°.5 complex, Sgr A, Sgr B, and Sgr C. When compared with the Milky Way at the same spatial resolution, the HCN luminosities in the central kiloparsec of six nearby starburst galactic nuclei are typically larger (factors of ~ 0.5-20), but the CO luminosities are only moderately larger (factors of ~ 0.5-5). Averaged over the central 200 pc, the HCN/CO intensity ratio in the starburst galaxies is larger than in the Milky Way, but a few starburst nuclei have similar or smaller HCN/CO intensity ratios when averaged over larger regions. The spatial extent of elevated HCN/CO ratios (¿0.08) typically encompasses ¿200 pc in the starburst galaxies but only ~50 pc in the Milky Way. These results suggest that starburst nuclei have larger masses of dense gas, higher average gas densities in the central 200 pc, and larger regions of enhanced gas density than the Galactic center. When averaged to 630 pc resolution, the HCN and CO spectra show two main velocity components: (1) a broad feature of width ~250 km s -1 centered on V LSR = 0 km s -1 and (2) a narrow feature of width ~90 km s -1 centered on V LSR = 50 km s -1 . For the broad component, the HCN/CO J = 1 ¿ 0 intensity ratio is nearly constant over the entire velocity range, and its small value (R ~ 0.03) is typical of that found in normal galaxies at several kiloparsecs spatial resolution. The gas in the broad velocity component is spatially extended and may represent diffuse molecular gas bound not to individual clouds but rather directly to the Galactic center potential well in a central disk or bar. Gas in the narrow velocity component is associated with the dense giant molecular clouds associated with the l = 1°.5 complex, Sgr A, B, and C. © The American Astronomical Society. All rights reserved. Printed in U.S.A.

Citations Scopus - 49
1996 Paglione TAD, Marscher AP, Jackson JM, Bertsch DL, 'Diffuse Gamma-ray emission from the starburst galaxy NGC 253', Astrophysical Journal, 460 295-302 (1996)

The starburst galaxy NGC 253 was observed with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) satellite. We obtain a 2 s uppe... [more]

The starburst galaxy NGC 253 was observed with the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory (CGRO) satellite. We obtain a 2 s upper limit to the ¿-ray emission above 100 MeV of 8 × 10 -8 photons cm -2 s -1 . Because of their large gas column densities and supernova rates, nearby starburst galaxies were predicted to have ¿-ray fluxes detectable by EGRET. Our nondetection of ¿-rays from NGC 253 motivates us to reexamine in detail the premise of supernova acceleration of cosmic rays and the effect of enhanced cloud densities, photon densities, and magnetic fields on the high-energy spectra of galaxies. By modeling the expected ¿-ray and synchrotron spectra from NGC 253, we find that up to 20% of the energy from supernovae is transferred to cosmic rays in the starburst, which is consistent with supernova acceleration models. Our calculations match the EGRET and radio data well with a supernova rate of 0.08 yr -1 , a magnetic field B ¿ 5 × 10 -5 G, a density n ~ 300 cm -3 , a photon density U ph ~ 200 eV cm -3 , and an escape timescale t 0 ¿ 10 Myr. © 1996. The American Astronomical Society. All rights reserved.

Citations Scopus - 59
1995 Paglione TAD, Jackson JM, Ishizuki S, Rieu NQ, 'First observations of extragalactic CS J = 1¿0', Astronomical Journal, 109 1716-1723 (1995)

We report the first detection of extragalactic CS J = 1¿0 emission. Because CS requires relatively high densities for thermal excitation (n H2 ¿10 ... [more]

We report the first detection of extragalactic CS J = 1¿0 emission. Because CS requires relatively high densities for thermal excitation (n H2 ¿10 4 cm -3 ), CS emission is expected from the denser core regions of molecular clouds. Using the Nobeyama 45 m telescope, we detected unresolved CS J = 1¿0 emission from the galaxies NGC 253, M82, NGC 6946, and IC 342, and obtained sensitive upper limits to the emission from Maffei 2 and M51. The CS J = 1¿0 line was also detected from NGC 253 with the Nobeyama Millimeter Array, and CS J = 3¿2 emission was mapped along the major axis of NGC 253 with the IRAM 30 m. Analysis of a single gas component CS excitation model implies that the cloud properties in these galaxies are very similar to those derived from similar CO studies, even though the critical density of a given line of CS is ~25 times higher. On the other hand, the cloud properties derived from CS studies are different from those derived from HCN, whose lines have critical densities ~13 times those of CS. The CS J = 1¿0 emission also correlates better with CO J = 1¿0 emission than with that of HCN. From theoretical CO/CS and HCN/CS J = 1¿0 line ratio calculations, we find that the observed HCN flux is lower than expected. The simplest explanation for this result is that the dense gas traced by HCN is concentrated in smaller regions than the more diffuse gas traced by CS and CO. Model calculations suggest that the total beam-averaged column density of H 2 traced by CO is greater than that traced by CS, which in turn is greater than the column density traced by HCN. Therefore, because CO, CS, and HCN apparently trace different physical regions in these galaxies, a single-component, uniform cloud model cannot simultaneously account for emission from species with different critical densities.

Citations Scopus - 17
1995 Jackson JM, Paglione TAD, Carlstrom JE, Nguyen-Q-Rieu, 'Submillimeter HCN and HCO

We report the first detection of extragalactic HCN and HCO + J = 4 ¿ 3 emission at ¿ = 850 µm. Because their critical densities (n ~ 10 7 cm -3 ) and excitation energies (E u... [more]

We report the first detection of extragalactic HCN and HCO + J = 4 ¿ 3 emission at ¿ = 850 µm. Because their critical densities (n ~ 10 7 cm -3 ) and excitation energies (E u ~ 45 K) are large, these lines trace dense, warm gas. We have detected HCN J = 4 ¿ 3 toward NGC 253, NGC 4945, and IC 342 and report a sensitive upper limit toward M82. We have also detected HCO + J = 4 ¿ 3 in both NGC 253 and M82. The HCN J = 4 ¿ 3/J = 1 ¿ 0 intensity ratio, which is larger for higher gas densities, is at least 12 times larger in NGC 253 than that in M82. Single-component gas excitation model calculations for NGC 253 show that, averaged over the central 300 pc, the molecular gas is warm (T ¿ 50 K) and dense (n ~ 5 × 10 5 cm -3 ). For M82, the lower HCN J = 4 ¿ 3/J = 1 ¿ 0 line ratio requires an average gas density at least 10 times smaller. In both M82 and NGC 253, the J = 4 ¿ 3/J = 1 ¿ 0 line ratios peak at the nucleus. Consequently, molecular gas is densest in the central 300 pc. The submillimeter HCN and HCO + lines reveal large differences in the dense gas properties, despite similar CO and far-infrared luminosities.

Citations Scopus - 39
1995 Paglione TAD, Tosaki T, Tosaki T, Jackson JM, 'The distribution of the dense clouds in the starburst nucleus of ngc 253', Astrophysical Journal, 454 L117-L120 (1995)

We present 2&quot; X 4&quot; resolution aperture synthesis observations of HCN J = 1 3 0 emission from the starburst galaxy NGC 253. Because of its large critical density, HCN tra... [more]

We present 2" X 4" resolution aperture synthesis observations of HCN J = 1 3 0 emission from the starburst galaxy NGC 253. Because of its large critical density, HCN traces dense (n H j 5 10 4 cm -3 ) molecular gas. The emission lies along a central bar in several unresolved (r = 30 pc) complexes. At 7" resolution the HCN J = 1 3 0/CO J = 1 3 0 intensity ratio, a measure of density, has a maximum value of 0.2. Excitation models suggest that the HCN-emitting clouds in NGC 253 have high temperatures (5100 K), densities (n^ - 104-105 cm-3), and beam-averaged column densities (NHj - 3-10 X 10 22 cm -2 ). We find a rough association between the star-forming regions in NGC 253 (radio continuum emission) and the molecular cloud complexes. Most of the radio sources have been interpreted as supernova remnants, but several have flat spectral indices typical of thermal emission from H II regions. The radio continuum sources have up to 100 times the radio luminosity of W49, the most luminous H II region complex in the Milky Way. The high cloud temperatures we infer for NGC 253 are most likely due to intense ultraviolet radiation from massive star formation and enhanced cosmic-ray heating by supernovae. The HCN distributions in NGC 253 and the Galactic center are remarkably similar when compared on the same spatial scale. They are asymmetric with longitude, and the peaks lie 100-200 pc away from the nucleus. However, the HCN luminosity and HCN/CO intensity ratio are several times higher in NGC 253 than in the Milky Way. Therefore, the average gas densities and the mass of dense gas are larger in the starburst nucleus of NGC 253. © 1995 IOP Publishing Ltd.

DOI 10.1086/309792
Citations Scopus - 51
1995 Kraemer KE, Jackson JM, 'Shock-excited NH

We report the discovery of four NH 3 (3, 3) masers in the NGC 6334 star formation region. The masers are found in two of the seven far-infrared continuum sources where high-mass ... [more]

We report the discovery of four NH 3 (3, 3) masers in the NGC 6334 star formation region. The masers are found in two of the seven far-infrared continuum sources where high-mass star formation is taking place in this molecular cloud. These masers occur at the ends of high-velocity molecular outflows; no maser emission was found near regions without high-velocity outflows. The NH 3 masers are not associated with any other type of maser. These results confirm that the NH 3 (3, 3) masers are caused by shocks and probably mark the location where the molecular outflow jet impinges upon the ambient medium.

Citations Scopus - 49
1994 Kerr RB, Noto J, Jackson JM, 'Deconvolution techniques applied to Fabry-Perot interferometry data', Proceedings of SPIE - The International Society for Optical Engineering, 2266 152-159 (1994)

Spectral line profiles from light, atomic gases in upper planetary atmospheres are commonly non-Maxwellian. The velocity distributions of these light gases are perturbed in comple... [more]

Spectral line profiles from light, atomic gases in upper planetary atmospheres are commonly non-Maxwellian. The velocity distributions of these light gases are perturbed in complex ways by atmospheric escape processes, by the paucity of thermalizing collisions, and by infrequent but important collisions with hot ions in the plasmasphere. It has long been recognized that the velocity distributions can be used to unfold the physical processes leading to atmospheric escape and to the partitioning of neutral gas trajectory classifications (ballistic, escaping, or satellite). Unfortunately, isolation of the velocity distribution from the measured emission line profile is not a simple matter, especially when neither of the velocity distributions are non- Maxwellian and when the instrument function used to measure the profile is also a complex function. We have experimented with several techniques to accurately retrieve the velocity distribution of atomic hydrogen in the earth's exosphere from the hydrogen Balmer-alpha (H a ) emission line profile measured with a Fabry-Perot interferometer. Although the derived velocity distribution remains subject to contamination of the measured emission by extraterrestrial and terrestrial sources, the technique to decompose the actual emission function from the combined instrument function plus emission function is established in this work - and is applicable to many other similar problems. In particular, two techniques are compared. First, a classical deconvolution technique is developed using objective, low-pass filtering. Second, a nonlinear deconvolution algorithm, commonly referred to as `CLEAN' by the radio astronomy community that developed it, is applied to the optical H a spectra. We find that this second technique is useful for an accurate isolation of the velocity distribution of atomic hydrogen in earth's exosphere, while the classical deconvolution is more useful for determining the full width at half maximum of the emission. The CLEAN technique does a superior job of isolating low signal-to-noise information in the emission profile wings, of particular interest for the derivation of the escaping atomic hydrogen population. It is particularly important that the CLEAN technique, when properly applied, is not susceptible to the addition of unrealistic information in the low signal-to-noise region of emission line extrema, whereas common deconvolution techniques are often quite suspect in these regions. Using this new technique and a new ability to ascribe hydrogen column abundance to H a brightness measurements, we are now poised to derive atomic hydrogen escape fluxes without dependence upon models of escape flux dynamics.

Citations Scopus - 2
1994 Ingalls JG, Bania TM, Jackson JM, 'Atomic carbon in the high-latitude molecular cloud MBM 12', Astrophysical Journal, 431 (1994)

We report the first detection of neutral atomic carbon emission from a high-latitude cloud. [C I] 492 GHz emission was detected from seven 13 CO cores in MBM 12, the nearest know... [more]

We report the first detection of neutral atomic carbon emission from a high-latitude cloud. [C I] 492 GHz emission was detected from seven 13 CO cores in MBM 12, the nearest known molecular cloud (d = 65 pc; b = -34°). There is strong evidence that the lines of [C I] and CO are formed in the same regions of the cloud. The average C/CO column density ratio is ¿0.7, which is a factor of ¿1.4-7 times higher than the same value measured toward dense Galactic photodissociation regions. Recent chemical models of translucent clouds with I UV = 1 Habing field can account for this ratio. Due to the proximity of MBM 12, we have discovered evidence for density clumps with sizes ¿2000 AU. The clumps contain H 2 masses of order the mass of Jupiter.

Citations Scopus - 20
1994 Poglitsch A, Geis N, Genzel R, Harris AI, Jackson JM, Krenz T, et al., 'Infrared observations of the center of the Galaxy', Infrared Physics and Technology, 35 419-429 (1994)

We report velocity resolved maps of the 158 µm [CII] fine structure line, of the 63 µm [OI] fine structure line, and of the 88 µm and 52µm [OIII] fine structure lines toward ... [more]

We report velocity resolved maps of the 158 µm [CII] fine structure line, of the 63 µm [OI] fine structure line, and of the 88 µm and 52µm [OIII] fine structure lines toward the center region of the Galaxy. Observations were carried out on board the Kuiper Airborne Observatory (KAO) using the MPE/UCB Far-infrared Imaging Fabry-Perot interferometer (FIFI). The angular resolution was 55¿ for the [CII] line and 22¿ for the oxygen lines. The [CII] emission, tracing the distribution of dense gas dissociated and partially ionized by far-UV photons, is strongest toward the rotating circumnuclear disk surrounding Sgr A West. A continuous bridge of [CII] line emission connects the Sgr A complex to the thermal radio filaments in the Radio Arc 10' north of the center, thus strongly suggesting a direct physical connection between the two. Outside the nuclear region, there is an anticorrelation between the [CII] intensity and the distribution of dense molecular material. The brightest emission occurs near the edges of the massive Galactic Center molecular clouds. We conclude that these clouds are predominantly ionized by external UV photons, partly from the Sgr A West region and partly from OB stars near the center. Our data indicate a physical connection between the center and the massive interstellar clouds in the surrounding 50 pc. Our 88 µm [OIII] maps of the "sickle" region within the Radio Arc and of the thermal arches approximately follow the thermal radio continuum. The line width varies substantially over the mapped area; we find a narrow (¿v ¿ 60km/s) and a broad (¿v ¿ 150km/s) component, with the broad component being more prominent in the sickle. From the 52 µm 88 µm line ratio we derive a density ¿ 300 cm -3 and a hydrogen column density ¿3 × 10 21 cm -1 in the [OIII] emitting medium. This rules out shocks or fast winds from stars as excitation mechaniµs. UV excitation, however, could produce the observed amount of O 2+ . About 10 to 100 O-stars would be required to ionize the gas in the "pistol/sickle" region. The broad linewidth could be due to interaction between the ionized gas and magnetic fields. Our maps of both [OIII] lines toward the circum-nuclear disk show the brightest emission from inside the central, "ionized cavity" in the molecular ring. The high 52 µm/88µm ratio requires a density = 10 4 cm -3 . The derived hydrogen column density, however, is low, which could be understood by a small ionization ratio N o2+ N o0 or a small beam filling factor of the ionized gas. We infer about 10 m¿ of ionized gas inside the cavity, with no indication of an additional low-density component. The [OI] data, on the other hand, reveal the presence of a large amount of neutral atomic gas inside the central cavity, and associated with the "northern" and "eastern" arms of ionized gas. We estimate about 300 M¿ of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuelear disk and falling from a gap in the 1.5pc ring into the central 0.5pc. We infer a mass inflow rate into the central parsec of 0.03M¿/year. © 1994.

DOI 10.1016/1350-4495(94)90099-X
Citations Scopus - 1
1994 Kraemer KE, Jackson JM, 'Radial Motions of Molecular Gas in the W49 Cloud Core', Astrophysical Journal Letters, 429 L37-L37 (1994)
1993 Jackson JM, Paglione TAD, Ishizuki S, Rieu NQ, 'Dense molecular gas toward the nucleus of the Seyfert galaxy NGC 1068', Astrophysical Journal, 418 (1993)

Using the Nobeyama Millimeter Array, we have imaged HCN J = 1 ¿ 0 emission at 7¿ (600 pc) resolution toward the Seyfert galaxy NGC 1068. Since HCN requires large densities (n cr... [more]

Using the Nobeyama Millimeter Array, we have imaged HCN J = 1 ¿ 0 emission at 7¿ (600 pc) resolution toward the Seyfert galaxy NGC 1068. Since HCN requires large densities (n crit ~4 × 10 6 cm -3 ) for excitation, its emission traces high-density molecular gas. About half of the total HCN emission comes from an unresolved source coincident with the Seyfert nucleus and half from the molecular ring at r = 1.6 kpc. The HCN J = 1 ¿ 0/CO J = 1 ¿ 0 integrated intensity (I = ¿ T B dV) ratio is I HCN /I CO = 0.4-0.9 for the nuclear region and is ~54 times smaller toward the molecular ring. The high HCN/CO intensity ratio toward the nucleus suggests that the HCN line is optically thick and that essentially all the molecular gas there is dense (n ¿ 10 5 cm -3 ). In contrast, the dense gas fraction or the opacity toward the molecular ring is significantly smaller. Optically thick HCN emission requires molecular hydrogen column densities N H2 ¿ 1.3 × 10 23 cm -2 , or A V ¿ 70 mag toward the nucleus. A velocity gradient is found for the nuclear region which is consistent with the measured linewidth only if the source size is r ~ 180 pc. If this velocity gradient represents rotation, then the HCN rotation axis (P.A. = 33°) is not coincident with the rotation axis of the galaxy but is instead aligned with the radio lobes and the ionization cones. Our observations support recent suggestions of a dense molecular torus which obscures the broad-line region in the Seyfert nucleus of NGC 1068.

Citations Scopus - 61
1993 Madden SC, Geis N, Genzel R, Herrmann F, Jackson JM, Poglitsch A, et al., '158 micron forbidden C II mapping of NGC 6946 - Probing the atomic medium', Astrophysical Journal, 407 579-579 (1993)
Citations Scopus - 88
1993 Jackson JM, Geis N, Genzel R, Harris AI, Madden S, Poglitsch A, et al., 'Neutral gas in the central 2 parsecs of the Galaxy', Astrophysical Journal, 402 173-184 (1993)

We have mapped the 1.1 mm HCN J = 3 ¿ 2 line with IRAM 30 m telescope at 12¿ resolution and the 63 µm [O I] 3 P 1 ¿ 3 P 2 line with the Kuiper Airborne Observatory at 20¿... [more]

We have mapped the 1.1 mm HCN J = 3 ¿ 2 line with IRAM 30 m telescope at 12¿ resolution and the 63 µm [O I] 3 P 1 ¿ 3 P 2 line with the Kuiper Airborne Observatory at 20¿ resolution within a projected distance of 2 pc of IRS 16/SgrA*. The HCN J = 3 ¿ 2 data, together with the previous HCN J = 1 ¿ 0 data of Güsten et al., indicate that the circumnuclear molecular material is arranged in several kinematically distinct streamers. We find some evidence for a nearly complete ring of clouds, including the so far missing eastern continuation of the "western arc," with an inclination of 70°, a radius of 1.5 pc, and a rotational velocity of 110 km s -1 . We infer a limit of about 19 km s -1 to any overall radial motion and systemic LSR velocity. A second feature with LSR velocities between +30 and +75 km s -1 lies northeast, east, and southeast of SgrA* and may represent material being fed from the massive +20/+50 km s -1 molecular clouds into the circumnuclear environment. West of SgrA* is a third feature with a radial velocity of about 70 km s -1 , approximately constant with position. It is probably unrelated to the ring, but instead a separate cloud with a large radial velocity. The [O I] data reveal the presence of a large amount of neutral atomic gas inside the central "ionized cavity," and associated with the "northern" and "eastern" arms of ionized gas. We estimate about 300 M ¿ of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor of 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuclear disk and falling from a gap in the 1.5 pc ring into the central 0.5 pc. We infer a mass inflow rate into the central parsec of 3 x 10 -2 M ¿ yr -1 . Almost all observed features of the spatial distribution of ionized, neutral atomic, and molecular gas in the central 2 pc of the Galaxy can be attributed to gas streamers rotating around or falling toward the Galactic center, and their illumination by several 10 6 L ¿ of ultraviolet radiation produced within a few arcseconds of IRS 16/SgrA*.

Citations Scopus - 145
1992 Nguyen-Q-Rieu, Jackson JM, Henkel C, Truong-Bach, Mauersberger R, 'A survey for extragalactic HCN and HCO

We have surveyed 15 galaxies for HCN and HCO + J = 1 ¿ 0 emission and detected HCN in 12 objects and HCO + in 11. Maps were made of IC 342, M51, and M83. Since the HCN and HCO ... [more]

We have surveyed 15 galaxies for HCN and HCO + J = 1 ¿ 0 emission and detected HCN in 12 objects and HCO + in 11. Maps were made of IC 342, M51, and M83. Since the HCN and HCO + lines require higher densities than CO J = 1 ¿ 0 for excitation, they probe denser gas. Strong HCO + J = 3 ¿ 2 emission has been found in NGC 253. Our new J = 1 ¿ 0 detections along with the J = 3 ¿ 2 data are used to estimate excitation temperatures and optical depths. We find T ex ~ 20 K for NGC 253 and T ex ~ 10 K for M82 and IC 342, which indicates a substantially larger kinetic temperature and/or volume density in NGC 253. The HCN/ HCO + line ratio varies from 0.5 to ¿4, independent of obvious star formation indicators. In M51 the inner spiral arms traced by CO are detected in HCN. In this galaxy the HCN emission peak does not coincide with the strongest CO peak but is instead located at the center of the nuclear CO ring. The maps of HCN and HCO + J = 1 ¿ 0 emission toward IC 342 are significantly different, especially at the periphery of the nuclear region. These results show that CO, HCN, and HCO + probe distinct physical regions. The HCN and HCO + J = 1 ¿ 0 emission is only weakly correlated with CO J = 1 ¿ 0 emission in a similar beam, suggesting that either the dense gas content or the HCN and HCO + abundances vary from galaxy to galaxy. In starburst and Seyfert galaxies, the emission of HCO + tends to be more tightly correlated with that of CO and with the nonthermal continuum emission of the central compact radio source. This indicates that the production and/or excitation of HCO + may be due to increased ionization by cosmic rays in the nuclear star-forming regions. The H 12 CN/H 13 CN and H 12 CO + /H 13 CO + intensity ratios in IC 342, NGC 253, and M82 are comparable to the 12 CO/ 13 CO ratios. This result is consistent with chemical models in the case of low electron density and weak line saturation.

Citations Scopus - 76
1992 Wild W, Harris AI, Eckart A, Genzel R, Graf UU, Jackson JM, et al., 'A multi-line study of the molecular interstellar medium in M 82's starburst nucleus', Astronomy & Astrophysics, 265 447-447 (1992)
1991 Poglitsch A, Beeman JW, Geis N, Genzel R, Haggerty M, Haller EE, et al., 'The MPE/UCB far-infrared imaging Fabry-Perot interferometer (FIFI)', International Journal of Infrared and Millimeter Waves, 12 859-884 (1991)

FIFI is an imaging spectrometer with two or three Fabry-Perot interferometers (FPI) in series for airborne astronomical observations in the far-infrared range (¿=40...200µm). It... [more]

FIFI is an imaging spectrometer with two or three Fabry-Perot interferometers (FPI) in series for airborne astronomical observations in the far-infrared range (¿=40...200µm). It employs 5×5 arrays of photoconducting detectors and offers spectral resolutions as small as 2km/s. Resolution and bandwidth can be set over a wide range to match a variety of astronomical sources. Cryogenic optics minimizes thermal background radiation and provides for in-flight step tunable spatial resolution. At 158 µm wavelength the background-limited NEP is 3 × 10 -15 W/QHz at 40 km/s resolution and with two FPI's; with three FPI's the expected NEP is =10 -15 WvHz at 5 km/s resolution. The frequency-chopping mode of the high-resolution Fabry-Perot allows for line detection in extended objects. Absolute internal flux calibration ensures adequate "flat fielding" of the array elements. © 1991 Plenum Publishing Corporation.

DOI 10.1007/BF01009647
Citations Scopus - 40
1991 Jackson JM, Eckart A, Cameron M, Wild W, Ho PTP, Pogge RW, Harris AI, 'The interstellar medium of the hot-spot galaxy NGC 2903', Astrophysical Journal, 375 105-115 (1991)

We report 21 cm radio continuum, Ha, IRAS, and J = 1 ¿ 0 and 2 ¿ 1 CO observations of the hot-spot galaxy NGC 2903. The radio continuum, which traces synchrotron emission, Ha, w... [more]

We report 21 cm radio continuum, Ha, IRAS, and J = 1 ¿ 0 and 2 ¿ 1 CO observations of the hot-spot galaxy NGC 2903. The radio continuum, which traces synchrotron emission, Ha, which traces thermal H II regions and CO, which traces neutral molecular gas, are distributed almost identically in a central bar. The CO data are consistent with emission from giant molecular clouds with properties typical of Milky Way clouds; that is, with large optical depths, excitation temperatures near 10 K, and sizes about 100 pc. If the standard CO luminosity to H 2 mass conversion factor holds in NGC 2903, then the central 770 pc contains about 3 x 10 8 M ¿ in molecular gas, comparable to the central 500 pc of the Galaxy, and a mass surface density of ~240 M ¿ pc -2 , comparable to Orion and the Galactic center. The star formation rate per unit area is enhanced by an order of magnitude in the nuclear region. The global far-infrared-to-CO luminosity ratio which is often interpreted as a measure of star formation efficiency, is typical of that for normal spirals. However the local FIR/CO ratio varies considerably. In particular, 100 µm emission extends well outside the molecular bar into a region of H I emission. We associate this emission with cirrus, which contributes about 30% of the global 100 µm flux. Where cirrus emission dominates, the FIR/CO ratio does not reflect the local star formation efficiency. The nuclear starburst in NGC 2903 may result from a large amount of molecular gas with normal star formation efficiency collected into a small nuclear region, due to streaming motions induced by the bar.

Citations Scopus - 31
1991 Stacey GJ, Townes CH, Poglitsch A, Madden SC, Jackson JM, Herrmann F, et al., 'The optical depth of the 158 micron [

We report the first detection of the F = 1 ¿ 0 hyperfine component of the 158 µm [ 13 C II] fine-structure line in the interstellar medium. A 12 point intensity map was obtained... [more]

We report the first detection of the F = 1 ¿ 0 hyperfine component of the 158 µm [ 13 C II] fine-structure line in the interstellar medium. A 12 point intensity map was obtained of the [ 13 C II] distribution over the inner 190¿ (R.A.) × 190¿ (decl.) regions of the Orion Nebula using an imaging Fabry-Perot interferometer. The [ 12 C II]/[ 13 C II] line intensity ratio varies significantly over the region mapped. It is highest (86 ± 9) in the core of the Orion H II region, and significantly lower (62 ± 7) in the outer regions of the map, reflecting higher optical depth in the [ 12 C II] line here. We suggest that this enhanced optical depth is the result of limb brightening of the optically thin [ 13 C II] line at the edges of the bowl-shaped H II region blister. If the 12 C/ 13 C abundance ratio is 43, the [ 12 C II] line in the inner regions of the Orion Nebula has a low optical depth: t 12 ~ 0.75 ± 0.25. The optical depth together with the large brightness temperature of the [ 12 C II] line (~160 K) requires that the excitation temperature of the 2 P 3/2 level be ~310 K, in very good agreement with the previous analysis of the physical conditions of the Orion interface region based on fine-structure line intensity ratios and photodissociation region models. If the 12 C/ 13 C abundance ratio is 67, the line optical depth is somewhat larger (t 12 ~ 1.85), and the transition excitation temperature is somewhat smaller (~190 K) than that predicted by these models. The present results therefore support values ~43 for the 12 C/ 13 C abundance ratio in the Orion Nebula.

Citations Scopus - 16
1991 Ho PTP, Ho LC, Szczepanski JC, Jackson JM, Armstrong JT, Barrett AH, 'A molecular gas streamer feeding the Galactic Centre', Nature, 350 309-312 (1991)

Radio maps of ammonia emission around the nucleus of the Milky Way reveal a long streamer of molecular gas connecting the well known circumnuclear 2-pc ring with molecular clouds ... [more]

Radio maps of ammonia emission around the nucleus of the Milky Way reveal a long streamer of molecular gas connecting the well known circumnuclear 2-pc ring with molecular clouds 10-20 pc further out. These clouds appear to be interacting with nearby supernova remnants, suggesting that the impact of the supernovae on the gas may have impelled some of it inwards, thus providing a source of fuel for the activity at the Galactic Centre. © 1991 Nature Publishing Group.

Citations Scopus - 48
1991 Eckart A, Cameron M, Jackson JM, Genzel R, Harris AI, Wild W, 'The molecular interstellar medium and the near-infrared continuum of the barred liner NGC 6764', Astrophysical Journal, 372 67-67 (1991)
Citations Scopus - 20
1991 Poglitsch A, Stacey GJ, Geis N, Haggerty M, Jackson JM, Rumitz M, et al., 'A survey of the 158 micron forbidden C II fine-structure line in the central 50 parsecs of the Galaxy', Astrophysical Journal Letters, 374 L33-L33 (1991)
Citations Scopus - 25
1991 Rieu NQ, Henkel C, Jackson JM, Mauersberger R, 'Detection of HNCO in external galaxies', Astronomy & Astrophysics (Letters), 241 L33-L33 (1991)
1990 Eckart A, Cameron M, Genzel R, Jackson JM, Rothermel H, Stutzki J, et al., 'Molecular absorption lines toward the nucleus of Centaurus A', The Astrophysical Journal: an international review of astronomy and astronomical physics, 365 522-522 (1990)
Citations Scopus - 26
1990 Ho PTP, Martin RN, Turner JL, Jackson JM, 'VLA imaging of extragalactic ammonia¿hot gas in the nucleus of IC 342', Astrophysical Journal Letters, 355 (1990)
Citations Scopus - 28
1989 Jackson JM, Snell RL, Ho PTP, Barrett AH, 'CO in Optically Selected Starburst Galaxies', The Astrophysical Journal: an international review of astronomy and astronomical physics, 337 680-680 (1989)
1989 Jackson JM, Rieu NQ, 'Aperture Synthesis Observations of Shocked SO2 in the bipolar nebula OH 231.8+4.2', Astrophysical Journal Letters, 335 (1989)
1989 Rieu NQ, Nakai N, Jackson JM, 'Dense Molecular Gas in Galaxies ¿ HCN, HCO+, and CS in M 82 and NGC 253', Astronomy & Astrophysics, 220 57-57 (1989)
1988 Lo KY, Tilanus R, Allen RJ, Wright MJH, Jackson JM, 'Gaseous Spiral Structure in M51', Lecture Notes in Physics, 315 439-439 (1988)
1988 Jackson JM, Ho PTP, 'Elongated CO structure in the starburst galaxy NGC 2146', Astrophysical Journal Letters, 324 (1988)
1988 Jackson JM, Ho PTP, Harschick AD, 'A circumstellar rotating disk surrounding NGC 6334 I', Astrophysical Journal Letters, 333 (1988)
1987 Welch WJ, Dreher JW, Jackson JM, Terebey S, Vogel SN, 'Star formation in W49A: Gravitational collapse of the molecular cloud core toward a ring of massive stars', Science, 238 1550-1555 (1987)

High-resolution molecular line and continuum radio images from the Hat Creek Radio Observatory and the Very Large Array suggest that the core of the W49A starforming region is und... [more]

High-resolution molecular line and continuum radio images from the Hat Creek Radio Observatory and the Very Large Array suggest that the core of the W49A starforming region is undergoing gravitational collapse. The radio continuum shows a 2-parsec ring of at least ten distinct ultracompact H-II regions, each associated with at least one O star. The ring is a region of large-scale, organized massive star formation. Recombination line velocities and HCO + excitation requirements indicate that the ring is rotating around 50,000 solar masses of material. Because the HCO + (1-0) line shows red-shifted absorption but blue-shifted emission, the molecular cloud core is believed to be collapsing toward the center of the ring. The HCO + radial velocities, as well as H-I, H 2 CO, and magnetic-field measurements, fit a simple model of inside-out gravitational collapse of a once magnetically supported cloud.

Citations Scopus - 82
1987 Feigelson ED, Jackson JM, Mathieu RD, Myers PC, Walter FM, 'An X-ray survey for pre-main-sequence stars in the Taurus-Auriga and Perseus molecular cloud complexes', The Astronomical Journal, 94 125-125 (1987)
1987 Jackson JM, Barrett AH, Armstrong JT, Ho PTP, 'Neutral Hydrogen in Starburst Galaxies', The Astronomical Journal, 93 53-53 (1987)
1985 Ho PTP, Jackson JM, Barrett AH, Armstrong JT, 'Interactions between the continuum sources in the galactic center and their immediate molecular environment', The Astrophysical Journal: an international review of astronomy and astronomical physics, 288 575-575 (1985)
1984 Jackson JM, Armstrong JT, Barrett AH, 'HNCO in Molecular Clouds', The Astrophysical Journal: an international review of astronomy and astronomical physics, 280 608-608 (1984)
1984 Stutzki J, Olberg M, Winnewisser G, Jackson JM, Barrett AH, 'NH3 hyperfine intensity anomalies', Astronomy & Astrophysics, 139 258-258 (1984)
Show 140 more journal articles

Conference (4 outputs)

Year Citation Altmetrics Link
2016 Jackson JM, Hogge T, Stephens I, Whitaker JS, 'RAMPS: The Radio Ammonia Mid-Plane Survey', American Astronomical Society (2016)
2016 Federrath C, Rathborne JM, Longmore SN, Kruijssen JMD, Bally J, Contreras Y, et al., 'The link between solenoidal turbulence and slow star formation in G0.253+0.016', Proceedings of the International Astronomical Union (2016)

© Copyright International Astronomical Union 2017. Star formation in the Galactic disc is primarily controlled by gravity, turbulence, and magnetic fields. It is not clear that t... [more]

© Copyright International Astronomical Union 2017. Star formation in the Galactic disc is primarily controlled by gravity, turbulence, and magnetic fields. It is not clear that this also applies to star formation near the Galactic Centre. Here we determine the turbulence and star formation in the CMZ cloud G0.253+0.016. Using maps of 3 mm dust emission and HNCO intensity-weighted velocity obtained with ALMA, we measure the volume-density variance s¿/¿0=1.3±0.5 and turbulent Mach number = 11±3. Combining these with turbulence simulations to constrain the plasma ß = 0.34±0.35, we reconstruct the turbulence driving parameter b=0.22±0.12 in G0.253+0.016. This low value of b indicates solenoidal (divergence-free) driving of the turbulence in G0.253+0.016. By contrast, typical clouds in the Milky Way disc and spiral arms have a significant compressive (curl-free) driving component (b > 0.4). We speculate that shear causes the solenoidal driving in G0.253+0.016 and show that this may reduce the star formation rate by a factor of 7 compared to nearby clouds.

DOI 10.1017/S1743921316012357
2005 Simon R, Jackson JM, Bania TM, Clemens DR, Heyer MH, 'The distribution of MSX infrared dark clouds in the inner Milky Way', Astronomische Nachrichten (2005)

Recent high resolution surveys with the ISO and MSX satellites have revealed a large number of Galactic clouds with significant extinction in the mid-infrared. These infrared dark... [more]

Recent high resolution surveys with the ISO and MSX satellites have revealed a large number of Galactic clouds with significant extinction in the mid-infrared. These infrared dark clouds (IRDCs) are characterized by their high masses, high column densities, and low temperatures, which makes them interesting candidates for clouds in the earliest stages of star formation. Little is known, however, about their origin and distribution in the Galaxy. We use mid-IR data acquired by the MSX satellite mission to identify and catalog IR dark cloud and core candidates in the first and fourth Galactic quadrants using a median filtering technique applied to the images. The catalog contains ~ 12000 objects. Their distribution is concentrated around the Galactic mid-plane and traces prominent bright star forming regions. Selected high contrast IR dark clouds in the first Galactic quadrant are correlated with 13 CO emission from the BU-FCRAO Galactic Ring Survey (GRS), a high resolution survey of 13 CO emission in the inner Milky Way. With the GRS it is possible to spectroscopically determine distances to the IR dark clouds and to derive physical parameters, such as sizes and masses, of the dark cores and their parental molecular clouds. Based on morphological correlation of mid-IR extinction and GRS 13 CO emission in velocity channel maps, we assign radial velocities to the IR dark clouds throughout the first Galactic quadrant. Assuming a flat rotation curve with (R ¿ , v 0 )=(8.5 kpc, 220 km s -1 ) and that the clouds are at the near kinematic distance, we break the distance ambiguity and determine their location in the Galaxy. We selected a total of 375 dark clouds in the GRS coverage of the first Galactic quadrant with contrasts higher than 20% and sizes larger than 1'. Of these, 318 have clean morphological matches with GRS molecular line emission in distinct velocity channels. The remaining dark clouds are either low contrast clouds, have only weak GRS emission counterparts or molecular line emission in more than one velocity channel, or no GRS emission at any velocity. The latter are often found close to bright H II regions and are probably holes in the MSX emission. The majority of IR dark clouds is found towards the 5 kpc Galactic Ring. High contrast IR dark clouds almost always have bright, extended 13 CO emission. Thus, in many cases, they are condensations inside larger Giant Molecular Clouds. Core masses are typically a few 100 to a few 1000 M ¿ . We therefore suggest that the condensations in the most massive IR dark clouds represent high mass proto-clusters, or OB-associations in the making.

1998 Swain MR, Bradford CM, Stacey GJ, Bolatto AD, Jackson JM, Savage ML, Davidson JA, 'Design of the South Pole imaging Fabry-Perot interferometer (SPIFI)', Proceedings of SPIE - The International Society for Optical Engineering (1998)

The South Pole Imaging Fabry-Perot Interferometer (SPIFI) is a direct detection, imaging, submillimeter spectrometer. The spectral resolving elements are a pair of cryogenic, scan... [more]

The South Pole Imaging Fabry-Perot Interferometer (SPIFI) is a direct detection, imaging, submillimeter spectrometer. The spectral resolving elements are a pair of cryogenic, scanning Fabry-Perot interferometers which use a free- standard Ni mesh for the etalon mirrors. The detectors for SPIFI are a 5 X 5 array of bolometers coupled to the focal plane with Winston cones. An adiabatic demagnetization refrigerator cools the bolometers to approximately 60 mK while a 3 He system operates simultaneously as a thermal guard. SPIFI is intended to operate on the ASO/RO submillimeter telescope at the South Pole and on the JCMT telescope on Mauna Kea and will be used to study the gas- phase reservoirs of carbon in star-forming regions in our own and near-by galaxies. SPIFI takes advantage of three things: (1) Advanced bolometers that achieve background limited performance at very high resolving powers. (2) The imaging capability and high spectral resolving power of Fabry-Perot interferometers. (3) The superb atmospheric transmission in submillimeter bands possible from the South Pole. The SPIFI uses state-of-the-art monolithic silicon bolometers fabricated at the NASA Goddard Space Flight Center. The cryogenic, scanning Fabry-Perots in SPIFI were designed and built at Cornell and are an evolution of the design used with great success for the Kuiper Wide Field Camera. The 1.7 m Antarctic Submillimeter Telescope/Remote Observatory exploits what is thought to be the best submillimeter observing site in the world. ©2003 Copyright SPIE - The International Society for Optical Engineering.

DOI 10.1117/12.317274
Citations Scopus - 11
Show 1 more conference
Edit

Grants and Funding

Summary

Number of grants 22
Total funding $7,281,363

Click on a grant title below to expand the full details for that specific grant.


20163 grants / $598,846

Radio Survey for Ammonia in the Mid-Plane of the Milky Way Galaxy$511,578

Stars form in dark dusty clouds. These stellar nurseries are literally shrouded in mystery. Dark, cold star-forming clouds pervade our Milky Way Galaxy. The investigators seek to understand how these clouds transform into clumps of stars. Some clouds collapse into clusters of hundreds of stars. The investigators will map a large swath of the Milky Way Galaxy, which contains thousands of these giant clouds. This map will reveal the location of these clouds, and establish their distance, structure, temperature, and internal motions. All of these measurements will be compared with theories to learn which of our ideas about star formation should be supported. By comparing younger clouds with older clouds, the investigators will learn how the clouds change as they form stars. Since our sun formed in this way, this study will shed light on the origin of the sun and the earth. The PI and a graduate student also will volunteer two weeks each summer to participate in the K-12 educational outreach programs (one day workshops and extended overnight "science camps") for students and teachers in West Virginia.

Funding body: National Science Foundation

Funding body National Science Foundation
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2016
Funding Finish 2019
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

Infall toward High-Mass Star-forming Clumps and Cores: The [O I] 63 um Line$48,193

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson, Howard Smith, Ian Stephens, Edward Chambers, Robert Simon, Jill Rathborne

Scheme USRA Stratospheric Airborne Observatory for Infrared Astronomy (SOFIA)
Role Lead
Funding Start 2016
Funding Finish 2018
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

Characterizing [C II] Emission from Galactic High‐‐‐Mass Star‐‐‐Forming Clumps$39,075

This grant supports observations on the SOFIA airborne observatory.  This projects aims to characterize the emission of the primary cooling line in galaxies---the [C II] 158 micron line---toward a sample of 10 Galactic clumps.  These clumps are very well characterized by the MALT90 molecular line survey.  The authors will determine whether the [C II] emission from entire galaxies can plausibly be attributed to the sum of the [C II] emission from thousands of dense, high-mass star-forming clumps.

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson, Howard Smith, Andres Guzman, Ian Stephens, Scott Whitaker, Jill Rathborne, E. Chambers, Robert Simon

Scheme USRA Stratospheric Airborne Observatory for Infrared Astronomy (SOFIA)
Role Lead
Funding Start 2016
Funding Finish 2017
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20141 grants / $30,000

CSIRO Distinguished Visitor$30,000

These funds supported a sabbatical year at CSIRO Australia Telescope National Facility headquarters in Marsfield, NSW.  The PI worked with CSIRO collaborators in support of the MALT90 survey projects.

Funding body: CSIRO - Commonwealth Scientific and Industrial Research Organisation

Funding body CSIRO - Commonwealth Scientific and Industrial Research Organisation
Project Team

James M. Jackson, Jill M. Rathborne

Scheme Distinguished Visitor Program
Role Lead
Funding Start 2014
Funding Finish 2015
GNo
Type Of Funding Aust Competitive - Commonwealth
Category 1CS
UON N

20121 grants / $650,309

MALT90: A Molecular Line Survey of High-Mass Star-Forming Regions$650,309

Dr. Jackson and his team work on the US portion of the Millimeter Astronomy Legacy Team 90 GHz Survey (MALT90). MALT90 is a large systematic molecular line survey of dense cores and exploits the capability of Australia's ATNF Mopra 22 m telescope for fast mapping and simultaneous imaging of 16 molecular lines near 90 GHz. This large, international project surveys molecular line emission from thousands of dense molecular cores which were also identified by the ATLASGAL 870-micron dust continuum survey. These molecular lines probe the physical, chemical, and evolutionary state in dense molecular cloud cores, which represent the early stages in the formation of high-mass stars. The target cores span the complete evolutionary ranges from pre-stellar cores, to protostellar cores, and on to H II regions.

This work processes and verifies the survey data. In addition, the PI and his team will determine the kinematic distances and Galactic distribution of dense cores, which probe the Milky Way's spiral structure and compare the extragalactic molecular line-infrared luminosity correlations with those in Galactic cores. Two graduate students will work on this research as part of their PhD theses. The survey data are available to the public via the CSIRO ATNF Online Telescope Archive (AOTA). MALT90 will have broad impact within the community as a valuable resource for dense core studies. It will provide a huge, important database and will identify dozens of interesting targets of high-mass star-forming core for follow up observations with ALMA.

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2012
Funding Finish 2015
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20111 grants / $410,459

Herschel Dust Temperatures of Star Forming Cores$410,459

The Herschel satellite observatory imaged Galactic clouds in their submillimeter, millimeter, and far-infrared dust emission.

This project aims to analyze the Herschel data toward a sample of well-charaterized, dense molecular clumps that host the early stages of high-mass star formation.  By characterizing the dust temperatues, we can estimate the evolutionary state of the clumps.  Coupled with kinematic distance information from our MALT90 molecular line survey we can then determine the mass, luminosity, column density, and temperature of these cores.  Our overarching goal is to determine how these clumps evolve from cold pre-stellar clumps and on to active star-forming clumps containing H II regions.

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Astrophysical Data Analysis Program
Role Lead
Funding Start 2011
Funding Finish 2015
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20091 grants / $351,058

Galactic Structures Using 2MASS Data$351,058

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Astrophysical Data Analysis Program
Role Lead
Funding Start 2009
Funding Finish 2014
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20081 grants / $845,525

Infrared Dark Clouds$845,525

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2008
Funding Finish 2011
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20062 grants / $366,167

Spitzer Cycle 3: Active Star Formation in Infrared Dark Clouds$202,350

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Spitzer Space Telescope
Role Lead
Funding Start 2006
Funding Finish 2009
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

Protostars in Infrared Dark Clouds$163,818

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Spitzer Space Telescope
Role Lead
Funding Start 2006
Funding Finish 2009
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20052 grants / $966,414

Release and Analysis of the Galactic Ring Survey$683,816

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2005
Funding Finish 2008
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

The Midcourse Space Experiment Extended Source Catalog$282,597

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Astrophysical Data Analysis Program
Role Lead
Funding Start 2005
Funding Finish 2008
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20044 grants / $570,631

The MSX Dark Cloud Catalog$254,903

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Astrophysical Data Analysis Program
Role Lead
Funding Start 2004
Funding Finish 2006
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

The SMC: A Template for the Primitive ISM$115,272

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Spitzer Space Telescope
Role Lead
Funding Start 2004
Funding Finish 2007
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

24 Micron Pilot Survey of the Galactic Plane$115,011

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Spitzer Space Telescope
Role Lead
Funding Start 2004
Funding Finish 2007
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

The Small Scale Structure of Cluster Forming Infrared Dark Clouds$85,444

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Spitzer Space Telescope
Role Lead
Funding Start 2004
Funding Finish 2007
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20021 grants / $65,563

REU Supplement: The Galactic Ring Survey$65,563

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2002
Funding Finish 2004
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

20013 grants / $1,673,428

The Galactic Ring Survey$781,510

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 2001
Funding Finish 2004
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

Infrared Studies of Star Forming Regions in the Galactic Ring Survey$761,919

Funding body: NASA (USA)

Funding body NASA (USA)
Project Team

James M. Jackson

Scheme Astrophysical Data Analysis Program
Role Lead
Funding Start 2001
Funding Finish 2004
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

Study of Future Prospects for Infrared Astronomy in Antarctica$130,000

Funding body: University of Chicago

Funding body University of Chicago
Project Team

James M. Jackson, Dan P. Clemens, Thomas M. Bania

Scheme Center for Astrophysical Research in Antarctica
Role Lead
Funding Start 2001
Funding Finish 2004
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

19982 grants / $752,961

The Milky Way Galactic Ring Survey$475,006

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson, Dan P. Clemens, Thomas M. Bania

Scheme Project Grant
Role Lead
Funding Start 1998
Funding Finish 2001
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N

The Dense Interstellar Medium in Dwarf Galaxies$277,955

Funding body: NSF

Funding body NSF
Project Team

James M. Jackson

Scheme Project Grant
Role Lead
Funding Start 1998
Funding Finish 2001
GNo
Type Of Funding International - Competitive
Category 3IFA
UON N
Edit

Research Supervision

Number of supervisions

Completed12
Current1

Current Supervision

Commenced Level of Study Research Title Program Supervisor Type
2014 PhD Ammonia in Dense Star-forming Clumps Astronomy, Boston University MA Sole Supervisor

Past Supervision

Year Level of Study Research Title Program Supervisor Type
2014 Masters Hyperfine Anomalies in the Ammonia (1,1) Line toward High-mass Star-Forming Clumps Astronomy, Boston University Sole Supervisor
2014 PhD Characterizing Molecular Clouds in the Earliest Phases of High-Mass Star Formation Astronomy, Boston University Principal Supervisor
2012 PhD Molecular Line Observations of Infrared Dark Clouds in the Galaxy Astronomy, Boston University Sole Supervisor
2009 PhD Galactic Structure and the Turbulent Properties of Molecular Clouds from the Galactic Ring Survey Astronomy, Boston University MA Sole Supervisor
2009 PhD Star Formation Activity in Cores Within Infrared Dark Clouds Astronomy, Boston University Sole Supervisor
2006 Masters Galactic Structure from the BU-FCRAO Galactic Ring Survey Astronomy, Boston University MA Sole Supervisor
2004 Masters The distances to molecular clumps using 2MASS Astronomy, Boston University MA Sole Supervisor
2003 PhD The Large Scale Galactic Structure of Star Formation Tracers Astronomy, Boston University MA Principal Supervisor
2003 PhD Mid-Infrared Observations of Photodissociation Regions and MIRSI: A Mid-Infrared Spectrometer and Imager Developed for Ground Based Ob serving Astronomy, Boston University MA Co-Supervisor
2000 PhD The Interstellar Medium in Low Metallicity Environments Astronomy, Boston University MA Sole Supervisor
1998 PhD The Effects of Massive Star Formation on the Interstellar Medium: Photodissociated and Molecular Gas in NGC 6334 Astronomy, Boston University Sole Supervisor
1997 PhD The Properties of the Dense Gas in the Nuclei of Spiral Galaxies Astronomy, Boston University MA Sole Supervisor
Edit

Research Opportunities

Astronomy PhD scholarships

Study molecular clouds and high-mass star formation

PHD

School of Mathematical and Physical Sciences

4/11/2016 - 5/11/2017

Contact

Professor James Jackson
University of Newcastle
School of Mathematical and Physical Sciences
j.jackson@newcastle.edu.au

Edit

News

PhD Scholarship: Astrophysics of high-mass star-formation

May 19, 2017

A PhD scholarship opportunity is available for two students to study the astrophysics of high-mass star formation under the supervision of  Professor James Jackson.

Professor James Jackson

Position

Head of School
School of Mathematical and Physical Sciences
School of Mathematical and Physical Sciences
Faculty of Science

Contact Details

Email j.jackson@newcastle.edu.au
Phone +61 (2) 4921 5424
Mobile +61 (0) 439 263 538
Fax +61 (2) 4921 6898

Office

Room v124
Building Mathematics Building.
Location Callaghan
University Drive
Callaghan, NSW 2308
Australia
Edit