The SEDIGISM survey: The influence of spiral arms on the molecular gas distribution of the inner Milky Way^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: dcolombo@mpifr-bonn.mpg.de
² School of Physics & Astronomy, Cardiff University, Queen’s Building, The Parade, Cardiff CF24 3AA, UK
³ Department of Physics, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
⁴ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
⁵ Laboratoire d’astrophysique de Bordeaux, CNRS, Univ. Bordeaux, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁶ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁷ Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
⁸ Green Bank Observatory, PO Box 2, Green Bank, WV 24944, USA
⁹ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
¹⁰ Department of Astronomy, University of Florida, 211 Bryant Space Science Center, Gainesville, FL 32611, USA
¹¹ Space Science Institute, 4765 Walnut St Suite B, Boulder, CO 80301, USA
¹² School of Science and Technology, University of New England, Armidale, NSW 2351, Australia
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Astronomy Department, Universidad de Chile, Casilla 36-D, Santiago, Chile
¹⁵ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK
¹⁶ University of Florida Department of Astronomy, Bryant Space Science Center Gainesville, FL 32611, USA
¹⁷ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea
¹⁸ Commissariat à l’énergie atomique et aux énergies alternatives, 91191 Gif-sur-Yvette, Saclay, France
¹⁹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Cologne, Germany
²⁰ IAPS-INAF, Via Fosso del Cavaliere, 100, 00133 Rome, Italy

Received: 11 May 2021
Accepted: 28 September 2021

Abstract

The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM (Structure, Excitation, and Dynamics of the Inner Galactic Interstellar Medium) survey that observed a large area of the inner Galaxy in the ¹³CO (2–1) line at an angular resolution of 28′′. We analyse the influences of the spiral arms by considering the features of the molecular gas emission as a whole across the longitude–velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of ~10⁵–10⁶ M_⊙ kpc⁻¹. This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.