Boston University-Five College Radio Astronomy Research Articles

STRUCTURED MOLECULAR GAS REVEALS GALACTIC SPIRAL ARMS

We explore the development of structures in molecular gas in the Milky Way by applying the analysis of the brightness distribution function (BDF) and the brightness distribution index (BDI) in the archival data from the Boston University-Five College Radio Astronomy Observatory 13CO J=1-0 Galactic Ring Survey. The BDI measures the fractional contribution of spatially confined bright molecular emission over faint emission extended over large areas. This relative quantity is largely independent of the amount of molecular gas and of any conventional, pre-conceived structures, such as cores, clumps, or giant molecular clouds. The structured molecular gas traced by higher BDI is located continuously along the spiral arms in the Milky Way in the longitude-velocity diagram. This clearly indicates that molecular gas changes its structure as it flows through the spiral arms. Although the high-BDI gas generally coincides with H II regions, there is also some high-BDI gas with no/little signature of ongoing star formation. These results support a possible evolutionary sequence in which unstructured, diffuse gas transforms itself into a structured state on encountering the spiral arms, followed by star formation and an eventual return to the unstructured state after the spiral arm passage.

The Red MSX Source survey: distribution and properties of a sample of massive young stars

The Red MSX Source (RMS) survey has identified a large sample of massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Using a recent catalogue of molecular clouds derived from the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (GRS), and by applying a Galactic scaleheight cut off of 120 pc, we solve the distance ambiguity for RMS sources located within 18\degr < |l| > 54\degr. These two steps yield kinematic distances to 291 sources out of a possible 326 located within the GRS longitude range. Combining distances and integrated fluxes derived from spectral energy distributions, we estimate luminosities to these sources and find that > 90% are indicative of the presence of a massive star. We find the completeness limit of our sample is ~10^4 Lsun, which corresponds to a zero age main sequence (ZAMS) star with a mass of ~12 Msun. Selecting only these sources, we construct a complete sample of 196 sources. Comparing the properties of the sample of young massive stars with the general population, we find the RMS-clouds are generally larger, more massive, and more turbulent. We examine the distribution of this sub-sample with respect to the location of the spiral arms and the Galactic bar and find them to be spatially correlated. We identify three significant peaks in the source surface density at Galactocentric radii of approximately 4, 6 and 8 kpc, which correspond to the proposed positions of the Scutum, Sagittarius and Perseus spiral arms, respectively. Fitting a scale height to the data we obtain an average value of ~29+-0.5 pc, which agrees well with other reported values in the literature, however, we note a dependence of the scale height on galactocentric radius with it increases from 30 pc to 45 pc between 2.5 and 8.5 kpc.

MOLECULAR CLOUDS AND CLUMPS IN THE BOSTON UNIVERSITY-FIVE COLLEGE RADIO ASTRONOMY OBSERVATORY GALACTIC RING SURVEY

The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO) Galactic Ring Survey (GRS) of 13CO 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 13CO Galactic emission (46'' angular resolution on a 22'' grid) and achieved a spectral resolution of 0.21 km s–1. Because the GRS uses 13CO, an optically thin tracer, rather than 12CO, 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 13CO 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.

RESOLUTION OF THE DISTANCE AMBIGUITY FOR GALACTIC H II REGIONS

We resolve the kinematic distance ambiguity for 266 inner Galaxy H II regions out of a sample of 291 using existing H I and 13CO sky surveys. Our sample contains all H II regions with measured radio recombination line emission over the extent of the 13CO Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey (18° < l < 55° and |b| < 1) and contains ultra compact (UC), compact, and diffuse H II regions. We use two methods for resolving the distance ambiguity for each H II region: H I emission/absorption (H I E/A) and H I self-absorption (H I SA). We find that the H I E/A and H I SA methods can resolve the distance ambiguity for 72% and 87% of our sample, respectively. When projected onto the Galactic plane, this large sample appears to reveal aspects of Galactic structure, with spiral arm-like features at Galactocentric radii of 4.5 and 6 kpc, and a lack of H II regions within 3.5 kpc of the Galactic center. Our H II regions are approximately in the ratio of 2 to 1 for far versus near distances. The ratio of far-to-near distances for UC H II regions is 2.2 to 1. Compact H II regions are preferentially at the near distance; their ratio of far-to-near distances is 1.6 to 1. Diffuse H II regions are preferentially at the far distance; their ratio of far-to-near distances is 3.8 to 1. This implies that the distinction between UC and compact H II regions is largely due to distance, and that the large angular size of diffuse H II regions is not solely due to proximity to the Sun.

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 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 ~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 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| < 1°, a total of 75.4 deg2. 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 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.

A Comparison of13CO and CS Emission in the Inner Galaxy

We analyze 2 deg2 of the Galactic plane surveyed in CS J = 2 → 1 and 13CO 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 13CO 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 13CO clumps with emission above an integrated intensity of 15 K km s-1 (greater than 37 σ). The fraction of those infrared point sources detected as bright CS clumps above 1 K km s-1 (greater than 3 σ) is only 35%. The CS/13CO intensity ratio can be used as a measure of gas excitation conditions. We compared the observed CS and 13CO line intensities of the entire 2 deg2 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) 13CO flux threshold, is Tmb(CS)/Tmb(13CO) = 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 13CO positions with flux above 3 σ 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 deg2 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.