[en] As the precursors to stellar clusters, it is imperative that we understand the distribution and physical properties of dense molecular gas clouds and clumps. Such a study has been done with the ground-based Bolocam Galactic Plane Survey (BGPS). Now the Herschel infrared GALactic plane survey (Hi-GAL) allows us to do the same with higher-quality data and complete coverage of the Galactic plane. We have made a pilot study comparing dense molecular gas clumps identified in Hi-GAL and BGPS, using six 2° × 2° regions centered at Galactic longitudes of $\mathrm{\ell <!--\; ???\; -->}={11}^{\circ <!--\; ???\; -->}$, 30°, 41°, 50°, 202°, and 217°. We adopted the BGPS methodology for identifying clumps and estimating distances, leading to 6198 clumps being identified in our substudy, with 995 of those having well-constrained distances. These objects were evenly distributed with Galactic longitude, a consequence of Hi-GAL being source confusion limited. These clumps range in mass from 10⁻² to 10⁵ M_⊙ and have heliocentric distances of up to 16 kpc. When clumps found in both surveys are compared, we see that distances agree within 1 kpc and ratios of masses are of the order of unity. This serves as an external validation for BGPS and instills confidence as we move forward to cataloging the clumps from the entirety of Hi-GAL. In addition to the sources that were in common with BGPS, Hi-GAL found many additional sources, primarily due to the lack of atmospheric noise. We expect Hi-GAL to yield 2 × 10⁵ clumps, with 20% having well-constrained distances, an order of magnitude above what was found in BGPS.

[en] ALMA cycle 3 observations of ¹²CO J = 3 → 2, ¹³CO J = 4 → 3, SiO J = 8 → 7, and HCN J = 5 → 4 are presented. Significant extended emission is detected in ¹²CO J = 3 → 2 with a morphology that is indicative of m = 2 tidal features, suggesting gas inflow. In addition, outflows for both nuclei are found in ¹²CO J = 3 → 2. Significant SiO absorption is detected in the western nucleus. HCN that is morphologically distinct from CO is detected in both nuclei. These observations are compared to non-LTE radiative transfer models created using the Line Modeling Engine for simple gas dynamics to gain insight into how physical parameters, such as rotational velocity, turbulent velocity, gas temperature, dust temperature, and gas mass, can reproduce the observed kinematic and spatial features. The eastern nucleus is found to be best modeled with an inclusion of a temperature asymmetry from one side of the disk to the other. It is also found that the western nucleus is optically thick even in the less abundant species of ¹³CO, absorbing significant amounts of continuum radiation.

[en] The Bolocam Galactic Plane Survey (BGPS) data for a 6 deg² region of the Galactic plane containing the Galactic center are analyzed and compared to infrared and radio continuum data. The BGPS 1.1 mm emission consists of clumps interconnected by a network of fainter filaments surrounding cavities, a few of which are filled with diffuse near-IR emission indicating the presence of warm dust or with radio continuum characteristic of H II regions or supernova remnants. New 350 μm images of the environments of the two brightest regions, Sgr A and B, are presented. Sgr B2 is the brightest millimeter-emitting clump in the Central Molecular Zone (CMZ) and may be forming the closest analog to a super star cluster in the Galaxy. The CMZ contains the highest concentration of millimeter- and submillimeter-emitting dense clumps in the Galaxy. Most 1.1 mm features at positive longitudes are seen in silhouette against the 3.6-24 μm background observed by the Spitzer Space Telescope. However, only a few clumps at negative longitudes are seen in absorption, confirming the hypothesis that positive longitude clumps in the CMZ tend to be on the near side of the Galactic center, consistent with the suspected orientation of the central bar in our Galaxy. Some 1.1 mm cloud surfaces are seen in emission at 8 μm, presumably due to polycyclic aromatic hydrocarbons. A ∼0.⁰2 (∼30 pc) diameter cavity and infrared bubble between l∼ 0.⁰0 and 0.⁰2 surround the Arches and Quintuplet clusters and Sgr A. The bubble contains several clumpy dust filaments that point toward Sgr A*; its potential role in their formation is explored. Bania's Clump 2, a feature near l = 3⁰-3.⁰5 which exhibits extremely broad molecular emission lines (ΔV> 150 km s^-1), contains dozens of 1.1 mm clumps. These clumps are deficient in near- and mid-infrared emission in the Spitzer images when compared to both the inner Galactic plane and the CMZ. Thus, Bania's Clump 2 is either inefficient in forming stars or is in a pre-stellar phase of clump evolution. The Bolocat catalog of 1.1 mm clumps contains 1428 entries in the Galactic center between l = 358.⁰5 and l = 4.⁰5 of which about 80% are likely to be within about 500 pc of the center. The mass spectrum above about 80 M_sun can be described by a power-law ΔN/ΔM = N₀ M ^{-2.14(+0.1,-0.4)}. The power-law index is somewhat sensitive to systematic grain temperature variations, may be highly biased by source confusion, and is very sensitive to the spatial filtering inherent in the data acquisition and reduction.

[en] 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 N₂H⁺) spectroscopic data. We combine literature studies of star formation tracers and dust temperatures within IRDCs with our search for ultracompact (UC) H II regions to discuss a possible evolutionary sequence for IRDC clumps. In addition, we perform an analysis of mass tracers in IRDCs and find that 8 μm extinction masses and 1.1 mm BGPS masses are complementary mass tracers in IRDCs except for the most active clumps (notably those containing UC H II 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 ¹³CO, HCO⁺, and N₂H⁺ 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 N₂H⁺ are brighter in active clumps, due to an increase in temperature and/or density. We report the identification of four UC H II regions embedded within IRDC clumps and find that UC H II regions are associated with bright (∼>1 Jy) 24 μm point sources, and that the brightest UC H II regions are associated with 'diffuse red clumps' (an extended enhancement at 8 μm). The broad stages of the discussed evolutionary sequence (from a quiescent clump to an embedded H II region) are supported by literature dust temperature estimates; however, no sequential nature can be inferred between the individual star formation tracers.

[en] We present Atacama Large Millimeter Array (ALMA) Cycle-0 observations of the CO J = 6–5 line in the advanced galaxy merger Arp 220.

[en] We present the first extragalactic detections of several CH rotational transitions in the far-infrared in four nearby galaxies, NGC 1068, Arp 220, M82, and NGC 253, using the Herschel Space Observatory. The CH lines in all four galaxies are a factor of 2-4 brighter than the adjacent HCN and HCO⁺ J = 6-5 lines (also detected in the same spectra). In the star-formation-dominated galaxies, M82, NGC 253, and Arp 220, the CH/CO abundance ratio is low (∼10^–5), implying that the CH is primarily arising in diffuse and translucent gas where the chemistry is driven by UV radiation as found in the Milky Way interstellar matter. In NGC 1068, which has a luminous active galactic nucleus (AGN), the CH/CO ratio is an order of magnitude higher, suggesting that CH formation is driven by an X-ray-dominated region (XDR). Our XDR models show that both the CH and CO abundances in NGC 1068 can be explained by an XDR-driven chemistry for gas densities and molecular hydrogen column densities that are well constrained by the CO observations. We conclude that the CH/CO ratio may a good indicator of the presence of AGN in galaxies. We also discuss the feasibility of detecting CH in intermediate- to high-z galaxies with ALMA.

[en] We present an unbiased census of deeply embedded protostars in Perseus, Serpens, and Ophiuchus, assembled by combining large-scale 1.1 mm Bolocam continuum and Spitzer Legacy surveys. We identify protostellar candidates based on their mid-infrared (mid-IR) properties, correlate their positions with 1.1 mm core positions from Enoch et al. in 2006 and 2007, and Young et al. in 2006, and construct well-sampled spectral energy distributions using our extensive wavelength coverage (λ = 1.25-1100 μm). Source classification based on the bolometric temperature yields a total of 39 Class 0 and 89 Class I sources in the three-cloud sample. We compare to protostellar evolutionary models using the bolometric temperature-luminosity diagram, finding a population of low-luminosity Class I sources that are inconsistent with constant or monotonically decreasing mass accretion rates. This result argues strongly for episodic accretion during the Class I phase, with more than 50% of sources in a 'sub-Shu' (dM/dt < 10^-6 M _sun yr^-1) accretion state. Average spectra are compared to protostellar radiative transfer models, which match the observed spectra fairly well in Stage 0, but predict too much near-IR and too little mid-IR flux in Stage I. Finally, the relative number of Class 0 and Class I sources is used to estimate the lifetime of the Class 0 phase; the three-cloud average yields a Class 0 lifetime of 1.7 ± 0.3 x 10⁵ yr, ruling out an extremely rapid early accretion phase. Correcting photometry for extinction results in a somewhat shorter lifetime (1.1 x 10⁵ yr). In Ophiuchus, however, we find very few Class 0 sources (N _Class0/N_ClassI ∼ 0.1-0.2), similar to previous studies of that cloud. The observations suggest a consistent picture of nearly constant average accretion rate through the entire embedded phase, with accretion becoming episodic by at least the Class I stage, and possibly earlier.

[en] The Bolocam Galactic Plane Survey (BGPS) is a 1.1 mm continuum survey of dense clumps of dust throughout the Galaxy covering 170 deg². We present spectroscopic observations using the Heinrich Hertz Submillimeter Telescope of the dense gas tracers, HCO⁺ and N₂H⁺ 3-2, for all 6194 sources in the BGPS v1.0.1 catalog between 7.°5 ≤ l ≤ 194°. This is the largest targeted spectroscopic survey of dense molecular gas in the Milky Way to date. We find unique velocities for 3126 (50.5%) of the BGPS v1.0.1 sources observed. Strong N₂H⁺ 3-2 emission (T _mb > 0.5 K) without HCO⁺ 3-2 emission does not occur in this catalog. We characterize the properties of the dense molecular gas emission toward the entire sample. HCO⁺ is very sub-thermally populated and the 3-2 transitions are optically thick toward most BGPS clumps. The median observed line width is 3.3 km s^–1 consistent with supersonic turbulence within BGPS clumps. We find strong correlations between dense molecular gas integrated intensities and 1.1 mm peak flux and the gas kinetic temperature derived from previously published NH₃ observations. These intensity correlations are driven by the sensitivity of the 3-2 transitions to excitation conditions rather than by variations in molecular column density or abundance. We identify a subset of 113 sources with stronger N₂H⁺ than HCO⁺ integrated intensity, but we find no correlations between the N₂H⁺/HCO⁺ ratio and 1.1 mm continuum flux density, gas kinetic temperature, or line width. Self-absorbed profiles are rare (1.3%)

[en] We present an expanded distance catalog for 1710 molecular cloud structures identified in the Bolocam Galactic Plane Survey (BGPS) version 2, representing a nearly threefold increase over the previous BGPS distance catalog. We additionally present a new method for incorporating extant data sets into our Bayesian distance probability density function (DPDF) methodology. To augment the dense-gas tracers (e.g., HCO⁺(3-2), NH₃(1,1)) used to derive line-of-sight velocities for kinematic distances, we utilize the Galactic Ring Survey (GRS) ¹³CO(1-0) data to morphologically extract velocities for BGPS sources. The outline of a BGPS source is used to select a region of the GRS ¹³CO data, along with a reference region to subtract enveloping diffuse emission, to produce a line profile of ¹³CO matched to the BGPS source. For objects with a HCO⁺(3-2) velocity, ≈95% of the new ¹³CO(1-0) velocities agree with that of the dense gas. A new prior DPDF for kinematic distance ambiguity (KDA) resolution, based on a validated formalism for associating molecular cloud structures with known objects from the literature, is presented. We demonstrate this prior using catalogs of masers with trigonometric parallaxes and H II regions with robust KDA resolutions. The distance catalog presented here contains well-constrained distance estimates for 20% of BGPS V2 sources, with typical distance uncertainties ≲ 0.5 kpc. Approximately 75% of the well-constrained sources lie within 6 kpc of the Sun, concentrated in the Scutum-Centaurus arm. Galactocentric positions of objects additionally trace out portions of the Sagittarius, Perseus, and Outer arms in the first and second Galactic quadrants, and we also find evidence for significant regions of interarm dense gas

[en] We present a new distance estimation method for dust-continuum-identified molecular cloud clumps. Recent (sub-)millimeter Galactic plane surveys have cataloged tens of thousands of these objects, plausible precursors to stellar clusters, but detailed study of their physical properties requires robust distance determinations. We derive Bayesian distance probability density functions (DPDFs) for 770 objects from the Bolocam Galactic Plane Survey in the Galactic longitude range 7.°5 ≤ l ≤ 65°. The DPDF formalism is based on kinematic distances, and uses any number of external data sets to place prior distance probabilities to resolve the kinematic distance ambiguity (KDA) for objects in the inner Galaxy. We present here priors related to the mid-infrared absorption of dust in dense molecular regions and the distribution of molecular gas in the Galactic disk. By assuming a numerical model of Galactic mid-infrared emission and simple radiative transfer, we match the morphology of (sub-)millimeter thermal dust emission with mid-infrared absorption to compute a prior DPDF for distance discrimination. Selecting objects first from (sub-)millimeter source catalogs avoids a bias towards the darkest infrared dark clouds (IRDCs) and extends the range of heliocentric distance probed by mid-infrared extinction and includes lower-contrast sources. We derive well-constrained KDA resolutions for 618 molecular cloud clumps, with approximately 15% placed at or beyond the tangent distance. Objects with mid-infrared contrast sufficient to be cataloged as IRDCs are generally placed at the near kinematic distance. Distance comparisons with Galactic Ring Survey KDA resolutions yield a 92% agreement. A face-on view of the Milky Way using resolved distances reveals sections of the Sagittarius and Scutum-Centaurus Arms. This KDA-resolution method for large catalogs of sources through the combination of (sub-)millimeter and mid-infrared observations of molecular cloud clumps is generally applicable to other dust-continuum Galactic plane surveys.