Context. Molecular lines and line ratios are commonly used to infer properties of extra-galactic star forming regions. The new generation of millimeter receivers almost turns every observation into a line survey. Full exploitation of this technical advancement in extra-galactic study requires detailed bench-marking of available line diagnostics. Aims. We aim to develop the Orion B giant molecular cloud (GMC) as a local template for interpreting extra-galactic molecular line observations. Methods. We use the wide-band receiver at the IRAM-30 m to spatially and spectrally resolve the Orion B GMC. The observations cover almost 1 square degree at 26 resolution with a bandwidth of 32 GHz from 84 to 116 GHz in only two tunings. Results. We introduce the molecular anatomy of the Orion B GMC, including relationships between line intensities and gas column density or far-UV radiation fields, and correlations between selected line and line ratios. We also obtain a dust-traced gas mass that is less than approximately one third the CO-traced mass, using the standard X CO conversion factor. The presence of over-luminous CO can be traced back to the dependence of the CO intensity on UV illumination. As a matter of fact, while most lines show some dependence on the UV radiation field, CN and C 2 H are the most sensitive. Moreover, dense cloud cores are almost exclusively traced by N 2 H + . Other traditional high-density tracers, such as HCN(1−0), are also easily detected in extended translucent regions at a typical density of ∼500 H 2 cm −3 . In general, we find no straightforward relationship between line critical density and the fraction of the line luminosity coming from dense gas regions. Conclusions. Our initial findings demonstrate that the relationships between line (ratio) intensities and environment in GMCs are more complicated than often assumed. Sensitivity (i.e., the molecular column density), excitation, and, above all, chemistry contribute to the observed line intensity distributions, and they must be considered together when developing the next generation of extra-galactic molecular line diagnostics of mass, density, temperature, and radiation field.

A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

Abstract. Collisional excitation of the λ21 cm hyperfine transition is not strong enough to thermalize it in warm neutral ("intercloud") interstellar gas, which we show by simultaneously solving the equations of ionization and collisional equilibrium under typical conditions. Coupling of the λ21 cm excitation temperature and local gas motions may be established by the Ly-α radiation field, but only if strong Galactic Ly-α radiation permeates the gas in question. The Ly-α radiation tends to impart to the gas its own characteristic temperature, which is determined by the range of gas motions that occur on the spatial scale of the Ly-α scattering. In general, the calculation of H I spin temperatures is a more difficult and interesting problem than might have been expected, as is any interpretation of H I spin temperature measurements.

Aims. We wish to separate and quantify the CO luminosity and CO-H 2 conversion factor applicable to diffuse but partially-molecular ISM when H 2 and CO are present but C + is the dominant form of gas-phase carbon. Methods. We discuss galactic lines of sight observed in Hi, HCO + and CO where CO emission is present but the intervening clouds are diffuse (locally A V < ∼ 1 mag) with relatively small CO column densities N CO < ∼ 2 × 10 16 cm −2 . We separate the atomic and molecular fractions statistically using E B−V as a gauge of the total gas column density and compare N H 2 to the observed CO brightness. Results. Although there are H 2 -bearing regions where CO emission is too faint to be detected, the mean ratio of integrated CO brightness to N H 2 for diffuse ISM does not differ from the usual value of 1 K km s −1 of integrated CO brightness per 2 × 10 20 H 2 cm −2 . Moreover, the luminosity of diffuse CO viewed perpendicular to the galactic plane is 2/3 that seen at the Solar galactic radius in surveys of CO emission near the galactic plane. Conclusions. Commonality of the CO-H 2 conversion factors in diffuse and dark clouds can be understood from considerations of radiative transfer and CO chemistry. There is unavoidable confusion between CO emission from diffuse and dark gas and misattribution of CO emission from diffuse to dark or giant molecular clouds. The character of the ISM is different from what has been believed if CO and H 2 that have been attributed to molecular clouds on the verge of star formation are actually in more tenuous, gravitationally-unbound diffuse gas.

Abstract. Using the Plateau de Bure Interferometer and IRAM 30 m Telescope, we observed λ2-3 mm absorption lines of CS, SO, SO2, H2S and HCS + from some of the diffuse clouds which occult our well-studied sample of compact extragalactic mm-wave continuum sources. Our observations of SO, H2S and HCS + represent the first detections of these species in diffuse clouds; SO2 was not detected at all. We find a typical value N(CS) ≈ 0.5-2.0 × 10 12 cm −2 which is actually much smaller than the values derived previously, along very different lines of sight, from interpretation of CS J = 2-1 emission lines. CS forms somewhat sluggishly and is occasionally absent even in features with appreciable N(HCO + ). + via electron recombination in cool, quiescent gas but the obvious gas-phase routes to formation of HCS + fail by factors of 25 or more. But for lines of sight where CS is found, N(CS)/N(HCO