SEDIGISM: Structure, excitation, and dynamics of the inner Galactic interstellar medium

Abstract

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 .

H II regions and high-mass starless clump candidates

“…3.3. (3) (Sub)millimeter molecular line data: Structure, Excitation, and Dynamics of the Inner Galactic InterStellar Medium survey (SEDIGISM; Schuller et al 2017) mapped the inner Galactic plane (−60 • ≤ l ≤ 18 • , |b| ≤ 0.5 • ) in the J = 2 − 1 rotational transition of 13 CO and C 18 O using the SHFI instrument on APEX with a spatial resolution of about 30 and a velocity resolution of 0.25 km s −1 (Schuller et al 2017). We used this survey to explore kinematics of the HMSCs.…”

Molecular envelope around the HII region RCW 120

“…We also use APEX archival data at a frequency of 220 GHz to reveal the large-scale distribution of molecular gas around RCW 120. The data were obtained as a part of the SEDIGISM project (E-193.C-0584A-2014, PI: F. Schuller Schuller et al 2017). The region around RCW 120 (source G348.25+0.38) was observed twice (in 2014 and 2015).…”

“…an order of magnitude higher than in our data from 2009 (see Table 1). Since only a portion of the sky with galactic latitude b ≤ 0.5 • was covered by Schuller et al (2017), there are no data for the north-west part of RCW 120. Analysis of the data cubes was performed using GILDAS 1 .…”

CHIMPS: physical properties of molecular clumps across the inner Galaxy

“…More recently the FOREST unbiased Galactic-plane Imaging survey with the Nobeyama 45-m telescope (FUGIN; Umemoto et al 2017) has been completed, providing a view of 12 CO, 13 CO, and C 18 O J=1-0 emission in the Northern Galactic plane at 20-arcsec resolution. The Southern Galaxy is also becoming increasingly well surveyed in CO, with coverage in the J=1-0 transition of 12 CO, 13 CO, and C 18 O from the Three-mm Ultimate Mopra Milky Way Survey (ThrUMMS; Barnes et al 2015) and the Mopra Southern Galactic Plane CO Survey (Braiding et al 2015, also in C 17 O), and the Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium survey (SEDIGISM; Schuller et al 2017) covering the J=2-1 transition of 13 CO and C 18 O . The coverage of congruent CO survey data in multiple isotopologues and transitions enables more complete and largescale analyses of the excitation conditions of molecular clouds than has ever been possible.…”