Edward Thomas Chambers

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13CO 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°-557 and a latitude range of |b| 40°. At the velocity resolution of 0.21 km s-1, the typical rms sensitivity is σ(T) ~ 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.

The Characterization and Galactic Distribution of Infrared Dark Clouds

Using 13CO 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 Galaxys most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of ~5 pc, peak column densities of ~1022 cm-2, LTE masses of ~5 ? 103 M?, and volume-averaged H2 densities of ~2 ? 103 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.

THE GALACTIC DISTRIBUTION OF INFRARED DARK CLOUDS

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 13CO 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 arm 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 13CO 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 same line of sight.

Massive Protostars in the Infrared Dark Cloud MSXDC G034.43+00.24

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 H2. In addition, the broad line widths (ΔV ~ 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.

A Catalog of Midcourse Space Experiment Infrared Dark Cloud Candidates

We use 8.3 um 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 um 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 sigma 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 infrared dark clouds. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the clouds average extinction, are found by iteratively fitting 2-dimensional elliptical Gaussians 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.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 σ 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 clouds 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.

THE DISTRIBUTION OF INFRARED DARK CLOUDS IN THE FIRST GALACTIC QUADRANT

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 13CO (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 13CO (1-0) and CS (2-1) lines. We determine that the Midcourse Space Experiment selected IRDCs are not a homogeneous population, and 13CO (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 13CO (1-0) maps. We find a strong peak in the Galactocentric IRDC mass surface density distribution at R Gal ~ 4.5 kpc.

THE NH3 HYPERFINE INTENSITY ANOMALY IN HIGH-MASS STAR-FORMING REGIONS

Anomalous ammonia (NH3) 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.

A Catalog ofMidcourse Space ExperimentInfrared Dark Cloud Candidates

We use 8.3 um 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 um 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 sigma 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 infrared dark clouds. For each IRDC, we also catalog cores. These cores, defined as localized regions with at least 40% higher extinction than the clouds average extinction, are found by iteratively fitting 2-dimensional elliptical Gaussians 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.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 σ 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 clouds 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.