Observations of interstellar dust are often used as a proxy for total gas column density N H . By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS (Green et al. 2018), with accurate (opacity-corrected) Hi column densities and newlypublished OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ 353 , reddening E(B−V )and the total proton column density N H in the range (1-30)×10 20 cm −2 , along sightlines with no molecular gas detections in emission. We derive an N H /E(B−V ) ratio of (9.4±1.6)×10 21 cm −2 mag −1 for purely atomic sightlines at |b|>5 • , which is 60% higher than the canonical value of Bohlin et al. (1978). We report a ∼40% increase in opacity σ 353 =τ 353 /N H , when moving from the low column density (N H <5×10 20 cm −2 ) to moderate column density (N H >5×10 20 cm −2 ) regime, and suggest that this rise is due to the evolution of dust grains in the atomic ISM. Failure to account for Hi opacity can cause an additional apparent rise in σ 353 , of the order of a further ∼20%. We estimate molecular hydrogen column densities N H2 from our derived linear relations, and hence derive the OH/H 2 abundance ratio of X OH ∼1×10 −7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H2 in the range N H2 ∼(0.1−10)×10 21 cm −2 . This suggests that OH may be used as a reliable proxy for H 2 in this range, which includes sightlines with both CO-dark and CO-bright gas.* Estimated from OH(1667) 3σ detection limits using Tex=3.5 K, FWHM=1 km/s and N OH /N H 2 =10 −7 (see Section 5).

Context. The origin and life-cycle of molecular clouds are still poorly constrained, despite their importance for understanding the evolution of the interstellar medium. Many large-scale surveys of the Galactic plane have been conducted recently, allowing for rapid progress in this field. Nevertheless, a sub-arcminute resolution global view of the large-scale distribution of molecular gas, from the diffuse medium to dense clouds and clumps, and of their relationship to the spiral structure, is still missing. Aims. We have carried out a systematic, homogeneous, spectroscopic survey of the inner Galactic plane, in order to complement the many continuum Galactic surveys available with crucial distance and gas-kinematic information. Our aim is to combine this data set with recent infrared to sub-millimetre surveys at similar angular resolutions. Methods. The SEDIGISM survey covers 78 deg 2 of the inner Galaxy (−60CO. This isotopologue of CO is less abundant than 12 CO by factors up to 100. Therefore, its emission has low to moderate optical depths, and higher critical density, making it an ideal tracer of the cold, dense interstellar medium. The data have been observed with the SHFI single-pixel instrument at APEX. The observational setup covers the 13 CO(2 -1) and C 18 O(2 -1) lines, plus several transitions from other molecules. Results. The observations have been completed. Data reduction is in progress, and the final data products will be made available in the near future. Here we give a detailed description of the survey and the dedicated data reduction pipeline. To illustrate the scientific potential of this survey, preliminary results based on a science demonstration field covering −20 • ≤ ≤ -18.5 • are presented. Analysis of the 13 CO(2 -1) data in this field reveals compact clumps, diffuse clouds, and filamentary structures at a range of heliocentric distances. By combining our data with data in the (1-0) transition of CO isotopologues from the ThrUMMS survey, we are able to compute a 3D realization of the excitation temperature and optical depth in the interstellar medium. Ultimately, this survey will provide a detailed, global view of the inner Galactic interstellar medium at an unprecedented angular resolution of ∼30 .

Context. Little is known about how high-mass stars form. Around 30% of the young high-mass stars in the Galaxy are observed at the edges of ionized (H ii) regions. Therefore these are places of choice to study the earliest stages of high-mass star formation, especially towards the most massive condensations. High-spatial resolution observations in the millimeter range might reveal how these stars form and how they assemble their mass. Aims. We want to study the fragmentation process down to the 0.01 pc scale in the most massive condensation (1700 M ) observed at the south-western edge of the H ii region RCW 120 where the most massive Herschel cores (∼124 M in average) could form high-mass stars. Methods. Using ALMA 3 mm continuum observations towards the densest and most massive millimetric condensation (Condensation 1) of RCW 120, we used the getimages and getsources algorithms to extract the sources detected with ALMA and obtained their physical parameters. The fragmentation of the Herschel cores is discussed through their Jeans mass to understand the properties of the future stars. Results. We extracted 18 fragments from the ALMA continuum observation at 3 mm towards 8 cores detected with Herschel, whose mass and deconvolved size range from 2 M to 32 M and from 1.6 mpc to 28.8 mpc, respectively. The low degree of fragmentation observed, regarding to the thermal Jeans fragmentation, suggests that the observed fragmentation is inconsistent with ideal gravitational fragmentation and other ingredients such as turbulence or magnetic fields should be added in order to explain it. Finally, the range of fragments' mass indicates that the densest condensation of RCW 120 is a favourable place for the formation of high-mass stars with the presence of a probable UCH ii region associated with the 27 M Fragment 1 of Core 2.

The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic Interstellar Medium) survey used the APEX telescope to map 84 deg2 of the Galactic plane between ℓ = −60○ and ℓ = +31○ in several molecular transitions, including 13CO (2 – 1) and C18O (2 – 1), thus probing the moderately dense (∼103 cm−3) component of the interstellar medium. With an angular resolution of 30″ and a typical 1σ sensitivity of 0.8–1.0 K at 0.25 km s−1 velocity resolution, it gives access to a wide range of structures, from individual star-forming clumps to giant molecular clouds and complexes. The coverage includes a good fraction of the first and fourth Galactic quadrants, allowing us to constrain the large scale distribution of cold molecular gas in the inner Galaxy. In this paper we provide an updated overview of the full survey and the data reduction procedures used. We also assess the quality of these data and describe the data products that are being made publicly available as part of this first data release (DR1). We present integrated maps and position-velocity maps of the molecular gas and use these to investigate the correlation between the molecular gas and the large scale structural features of the Milky Way such as the spiral arms, Galactic bar and Galactic centre. We find that approximately 60 per cent of the molecular gas is associated with the spiral arms and these appear as strong intensity peaks in the derived Galactocentric distribution. We also find strong peaks in intensity at specific longitudes that correspond to the Galactic centre and well known star forming complexes, revealing that the 13CO emission is concentrated in a small number of complexes rather than evenly distributed along spiral arms.

Context. The edges of ionized (H II) regions are important sites for the formation of (high-mass) stars. Indeed, at least 30% of the Galactic high-mass-star formation is observed there. The radiative and compressive impact of the H II region could induce star formation at the border following different mechanisms such as the collect and collapse or the radiation-driven implosion (RDI) models and change their properties. Aims. We aim to study the properties of two zones located in the photo dissociation region (PDR) of the Galactic H II region RCW 120 and discuss them as a function of the physical conditions and young star contents found in both clumps. Methods. Using the APEX telescope, we mapped two regions of size 1.5′ × 1.5′ toward the most massive clump of RCW 120 hosting young massive sources and toward a clump showing a protrusion inside the H II region and hosting more evolved low-mass sources. The 12CO (J = 3−2), 13CO (J = 3−2) and C18O (J = 3−2) lines observed, together with Herschel data, are used to derive the properties and dynamics of these clumps. We discuss their relation with the hosted star formation. Results. Assuming local thermodynamic equilibrium, the increase of velocity dispersion and Tex are found toward the center of the maps, where star-formation is observed with Herschel. Furthermore, both regions show supersonic Mach numbers (7 and 17 in average). No substantial information has been gathered about the impact of far ultraviolet radiation on C18O photodissociation at the edges of RCW 120. The fragmentation time needed for CC to be at work is equivalent to the dynamical age of RCW 120 and the properties of region B are in agreement with bright-rimmed clouds. Conclusions. Although conclusions from this fragmentation model should be taken with caution, it strengthens the fact that, together with evidence of compression, CC might be at work at the edges of RCW 120. Additionally, the clump located at the eastern part of the PDR is a good candidate pre-existing clump where star-formation may be induced by the RDI mechanism.