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We report the first uniform and systematic study of dust and molecular gas in nearby molecular clouds. We use surveys of dust extinction and emission to determine the opacity and map the distribution of the dust within a dozen local clouds in order to derive a uniform set of basic cloud properties. We find: 1) the average dust opacity $\langle\kappa_{d,353}\rangle = 0.8\ {\rm cm^{2}\, g^{-1}}$ with variations of a factor of $\sim$ 2 between clouds, 2) cloud PDFs are exquisitely described by steeply falling power-laws with a narrow range of slope, and 3) a tight $M_{\rm GMC} \sim R_{\rm GMC}^2$ scaling relation for the cloud sample, indicative of a cloud population with an exactingly constant average surface density above a common fixed boundary. We compare these results to uniformly analyzed CO surveys. We measure the CO mass conversion factors and assess the efficacy of CO for tracing the physical properties of molecular clouds. We find $\langle \alpha_{\rm CO}\rangle = 4.31 \pm 0.67$ M$_\odot$ (K km s$^{-1}$ pc$^2$)$^{-1}$ (corresponding to $X_{\rm CO}$ = 1.97 $\times$ 10$^{20}$ cm$^{-2}$(K km s$^{-1}$)$^{-1}$). We demonstrate that CO observations are a poor tracer of column density and structure on sub-cloud spatial scales. On cloud scales, CO observations can provide measurements consistent with those of the dust, provided data are analyzed in a similar, self-consistent fashion. Measurements of average GMC surface density are sensitive to choice of cloud boundary. Care must be exercised to adopt common fixed boundaries when comparing surface densities for cloud populations within and between galaxies., Comment: 27 pages, 19 figures, 5 tables. Accepted ApJL. v2 Corrected bad coordinates on Appendix C maps. Brought text into alignment with final ApJL version

We have complemented existing observations of HI absorption with new observations of HCO$^+$, C$_2$H, HCN, and HNC absorption from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the direction of 20 background radio continuum sources with $4^\circ \leq |b| \leq 81^\circ$ to constrain the atomic gas conditions that are suitable for the formation of diffuse molecular gas. We find that these molecular species form along sightlines where $A_V \gtrsim 0.25$, consistent with the threshold for the HI-to-H$_2$ transition at solar metallicity. Moreover, we find that molecular gas is associated only with structures that have an HI optical depth $> 0.1$, a spin temperature $< 80$ K, and a turbulent Mach number $\gtrsim 2$. We also identify a broad, faint component to the HCO$^+$ absorption in a majority of sightlines. Compared to the velocities where strong, narrow HCO$^+$ absorption is observed, the HI at these velocities has a lower cold neutral medium (CNM) fraction and negligible CO emission. The relative column densities and linewidths of the different molecular species observed here are similar to those observed in previous experiments over a range of Galactic latitudes, suggesting that gas in the solar neighborhood and gas in the Galactic plane are chemically similar. For a select sample of previously-observed sightlines, we show that the absorption line profiles of HCO$^+$, HCN, HNC, and C$_2$H are stable over periods of $\sim 3$ years and $\sim 25$ years, likely indicating that molecular gas structures in these directions are at least $\gtrsim 100$ AU in size, 31 pages, 12 figures, 8 tables. Accepted to ApJ

We present a very large extension of the Galactic plane CO survey of Dame et al. to the entire northern sky (δ > −17°). The extension was carried out with the same telescope as was used for the plane survey, the CfA 1.2 m, and perfectly meshes with its irregular boundaries in latitude. A total of 382,202 CO(1–0) spectra uniformly sample the high-latitude sky with a true-angle spacing of 0.°25 or better. The final reduced and folded spectra have a uniform sensitivity of 0.18 K in 0.65 km s−1 channels and provide a velocity coverage of ±47.1 km s−1. We describe the observational techniques and the data reduction and provide various summary maps of the spatial and velocity distributions of CO emission over the northern sky, and a catalog of the molecular clouds we found there. We also describe the CO spectral line data cubes that we have made available online.

We compare observations of HI from the Very Large Array (VLA) and the Arecibo Observatory and observations of HCO$^+$ from the Atacama Large Millimeter/submillimeter Array (ALMA) and the Northern Extended Millimeter Array (NOEMA) in the diffuse ($A_V\lesssim1$) interstellar medium (ISM) to predictions from a photodissociation region (PDR) chemical model and multi-phase ISM simulations. Using a coarse grid of PDR models, we estimate the density, FUV radiation field, and cosmic ray ionization rate (CRIR) for each structure identified in HCO$^+$ and HI absorption. These structures fall into two categories. Structures with $T_s40~\mathrm{K}$, mostly with $N(\mathrm{HCO^+})\gtrsim10^{12}~\mathrm{cm^{-2}}$, are consistent with high density, FUV radiation field, and CRIR models, characteristic of environments close to massive star formation. The latter are also found in directions with a significant fraction of thermally unstable HI. In at least one case, we rule out the PDR model parameters, suggesting that alternative mechanisms (e.g., non-equilibrium processes like turbulent dissipation and/or shocks) are required to explain the observed HCO$^+$ in this direction. Similarly, while our observations and simulations of the turbulent, multi-phase ISM agree that HCO$^+$ formation occurs along sightlines with $N(\mathrm{HI})\gtrsim10^{21}~\mathrm{cm^{-2}}$, the simulated data fail to explain HCO$^+$ column densities $\gtrsim\rm{few}\times10^{12}~\mathrm{cm^{-2}}$. Since a majority of our sightlines with HCO$^+$ had such high column densities, this likely indicates that non-equilibrium chemistry is important for these lines of sight., 15 pages, 3 figures, 1 table. Accepted to ApJ

The Columbia - U. de Chile CO Survey of the Southern Milky Way is used for separating the CO(1-0) emission of the fourth Galactic quadrant within the solar circle into its dominant components, giant molecular clouds (GMCs). After the subtraction of an axisymmetric model of the CO background emission in the inner Southern Galaxy, 92 GMCs are identified, and for 87 of them the two-fold distance ambiguity is solved. Their total molecular mass is M(H2) = 1.14 +/- 0.05 x 10^8 Msolar accounting for around 40% of the molecular mass estimated from an axisymmetric analysis of the H2 volume density in the Galactic disk (Bronfman et al. 1988b) M(H2) disk = 3.03 x 10^8 Msolar. The large scale spiral structure in the Southern Galaxy, within the solar circle, is traced by the GMCs in our catalog; 3 spiral arm segments: the Centaurus, Norma, and 3-kpc expanding arm are analyzed. After fitting a logarithmic spiral arm model to the arms, tangent directions at 310{\deg}, 330{\deg}, and 338{\deg}, respectively, are found, consistent with previous values from the literature. A complete CS(2-1) survey toward IRAS point-like sources with FIR colors characteristic of UC HII regions is used to estimate the massive star formation rate per unit H2 mass (MSFR), and the massive star formation efficiency for GMCs. The average MSFR for GMCs is 0.41 +/- 0.06 Lsolar/Msolar, and for the most massive clouds in the Norma arm it is 0.58 +/- 0.09 Lsolar/Msolar. Massive star formation efficiencies of GMCs are on average 3% of their available molecular mass.

The near-infrared region of the spectrum is a particularly advantageous window for observing the distribution of old, evolved stars in the galactic disk and bulge. These stars are important because they provide an excellent tracer of the overall stellar mass distribution. At shorter wavelengths extinction is a serious problem, and at longer wavelengths the flux is dominated by dust emission. A summary of the large-scale diffuse near-infrared observations of the Galaxy is presented, as is a summary of the results obtained from these data on the structure of the galactic disk and bulge. The importance of combining CO and near-infrared maps of similar resolution to determine a three-dimensional model of galactic extinction is demonstrated. The Spacelab-2 Infrared Telescope (IRT) data are used in conjunction with a proposed galactic model to make preliminary measurements of the global scale parameters of the Galaxy.

Over the past four years, Pat Thaddeus and I have been using the CfA 1.2 m telescope to carry out a new CO survey of the first Galactic quadrant with a sensitivity per solid angle 3–10 times higher than previous CO surveys of the plane, and 16 times higher than the only previous extensive wide-latitude survey (Dame et al. 1987). Since a preliminary purpose of this survey is the study of molecular gas out of the plane in the inner Galaxy, the observations have so far been confined mainly to several vertical strips across the plane covering a few degrees in Galactic longitude and 6°–8° in Galactic latitude; details of this on-going survey are given in Dame & Thaddeus (1995).

It is now well established that molecular clouds are an important part of the interstellar medium, containing much or most of the dense, cold component of the gas, and producing the massive stars and supernovae responsible for the diffuse, hot component. It would therefore appear essential in formulating a complete picture of the local interstellar medium to have some knowledge of the distribution and properties of nearby molecular clouds. To this end we have used the Goddard-Columbia 1.2-meter telescope to carry out a wide latitude, low angular resolution survey of CO along most of the first galactic quadrant and a small part of the second. The survey is uniform and fully sampled in galactic longitude from 12° to 100°, and in latitude from -5° to +6°, with extensions to as high as +11° to include specific dark clouds; the total area surveyed, 1128 deg2, is much larger than any region previously studied in CO or any other interstellar molecule. In order to fully sample such a large area in a reasonable amount of time, angular resolution has been sacrificed to coverage and speed; an angular resolution of 1° was obtained synthetically by simply scanning a square 8 x 8 raster of points separated by 1/8°, the size of the primary beam, and summing the resulting 64 spectra at the end of an observation.

New date on the distribution of molecular hydrogen in the first and second quadrants allow a detailed analysis of the relationship between the column density of gas (in all major forms) and the intensity of cosmic gamma-rays in the Galaxy. It is concluded that most of the features observed in the gamma-ray map can be explained in terms of cosmic-ray interactions with the gas in the ISM, with molecular hydrogen playing an important role. The cosmic-ray intensity in the important production regions appears to be roughly independent of galactocentric distance, R, for l:65-100 deg and to fall off slowly with R for l:100-180 deg.

The discrepancy between observed and predicted γ-ray emission toward the Galactic Center is attributed to a unique population of wide-line molecular clouds. The most prominent objects of this class show evidence of rotation and a significant stellar population. The observed 12CO emission traces the gravitational field produced primarily by stars, not molecular gas.

Recently, the HESS array has reported the detection of gamma-ray emission above a few hundred GeV from eight new sources located close to the Galactic Plane. The source HESS J1834-087 is spatially coincident with SNR G23.3-0.3 (W41). Here we present MAGIC observations of this source, resulting in the detection of a differential gamma-ray flux consistent with a power law, described as dN/(dA dt dE) = (3.7 +/- 0.6)*10^(-12) (E/TeV)^(-2.5 +/- 0.2) \ cm^(-2)s^(-1)TeV^(-1). We confirm the extended character of this flux. We briefly discuss the observational technique used, the procedure implemented for the data analysis, and put this detection in the perspective of the molecular environment found in the region of W41. We present 13CO and 12CO emission maps showing the existence of a massive molecular cloud in spatial superposition with the MAGIC detection., Comment: Accepted by ApJ Letters