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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121, Bonn, Germany,
e-mail: schuller@mpifr-bonn.mpg.de
² European Southern Observatory, Alonso de Cordova 3107, Casilla 19001 Santiago 19, Chile
³ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
⁴ School of Physics, University of Exeter, Stocker Road, Exeter, EX4 4QL, UK
⁵ Astronomy Department, University of Florida, PO Box 112055, Gainesville, FL 32611, USA
⁶ School of Science and Technology, University of New England, NSW 2351 Armidale, Australia
⁷ Astronomy Department, University of Wisconsin, 475 North Charter St., Madison, WI 53706, USA
⁸ West Virginia University, Department of Physics & Astronomy, PO Box 6315, Morgantown, WV 26506, USA
⁹ Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Université Bordeaux, LAB, CNRS, UMR 5804, 33270 Floirac, France
¹² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹⁴ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool, L3 5RF, UK
¹⁵ National Astronomical Observatory of Japan, Chile Observatory, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
¹⁶ Department of Physics, Faculty of Science, Hokkaido University, 060-0810 Sapporo, Japan
¹⁷ Laboratoire d’Astrophysique de Marseille, Aix Marseille Université, CNRS, UMR 7326, 13388 Marseille, France
¹⁸ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁹ Istituto di Astrofisica e Planetologia Spaziali, INAF, via Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 13 May 2016
Accepted: 26 December 2016

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 relationshipto 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 deg² of the inner Galaxy (−60°≤ℓ≤ 18°, |b|≤ 0.5°) in the J = 2–1 rotational transition of ¹³CO. This isotopologue of CO is less abundant than ¹²CO 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 ¹³CO(2−1) and C¹⁸O(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°≤ℓ ≤ −18.5° are presented. Analysis of the ¹³CO(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′′.