Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)

III. ¹³CO/C¹⁸O ratio variations across the M51 galaxy

¹ Argelander Institute for Astronomy (AIfA), University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain
⁶ European Southern Observatory, Karl-Schwarzschild Straße 2, D-85748 Garching bei München, Germany
⁷ IRAM, 300 Rue de la Piscine, 38400 Saint Martin d’Hères, France
⁸ Sorbonne Université, Observatoire de Paris, Université PSL, École normale supérieure, CNRS, LERMA, F-75005 Paris, France
⁹ Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁰ Cardiff Hub for Astrophysics Research & Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, Cardiff CF24 3AA, UK
¹¹ Instituto de Astronomía, Universidad Nacional Autónoma de México, Ap. 70-264, 04510 CDMX, Mexico
¹² Department of Physics, Tamkang University, No. 151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
¹³ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author: igalic@astro.uni-bonn.de

Received: 22 December 2024
Accepted: 30 July 2025

Abstract

Context. CO isotopologues are common tracers of the bulk molecular gas in extragalactic studies, providing insights into the physical and chemical conditions of the cold molecular gas, a reservoir for star formation.

Aims. Since star formation occurs within molecular clouds, mapping CO isotopologues on the scale of clouds is important to understanding the processes driving star formation. However, achieving this mapping at such scales is challenging and time-intensive. The Surveying the Whirlpool Galaxy at Arcseconds with NOEMA (SWAN) survey addresses this by using the Institut de radioastronomie millimétrique (IRAM) NOrthern Extended Millimeter Array (NOEMA) to map the ¹³CO(1−0) and C¹⁸O(1−0) isotopologues, alongside several dense gas tracers, in the nearby star-forming galaxy M51 at high sensitivity and spatial resolution (≈125 pc).

Methods. We examine the ¹³CO(1−0) to C¹⁸O(1−0) line emission ratio as a function of galactocentric radius and star formation rate surface density to infer how different chemical and physical processes affect this ratio at cloud scales across different galactic environments: nuclear bar, molecular ring, and northern and southern spiral arms.

Results. In line with previous studies conducted at kiloparsec scales for nearby star-forming galaxies, we find a moderate positive correlation with galactocentric radius and a moderate negative correlation with star formation rate surface density across the field of view (FoV), with slight variations depending on the galactic environment.

Conclusions. We propose that selective nucleosynthesis and changes in the opacity of the gas are the primary drivers of the observed variations in the ratio.