Wide-field CO isotopologue emission and the CO-to-H₂ factor across the nearby spiral galaxy M101

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: jakob.denbrok@gmail.com
² Center for Astrophysics, Harvard & Smithsonian, 60 Garden St., 02138 Cambridge, MA, USA
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁴ Center for Astrophysics & Space Sciences, Department of Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
⁵ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
⁶ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁸ 4-183 CCIS, University of Alberta, Edmonton, Alberta T6G 2E1, Canada
⁹ Institute of Astronomy and Astrophysics, Academia Sinica, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
¹⁰ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
¹¹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹² Institüt für Theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert-Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹³ Cosmic Origins Of Life (COOL) Research DAO, coolresearch.io
¹⁴ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁵ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

Received: 18 December 2022
Accepted: 1 February 2023

Abstract

Carbon monoxide (CO) emission constitutes the most widely used tracer of the bulk molecular gas in the interstellar medium (ISM) in extragalactic studies. The CO-to-H₂ conversion factor, α_12CO(1−0), links the observed CO emission to the total molecular gas mass. However, no single prescription perfectly describes the variation of α_12CO(1−0) across all environments within and across galaxies as a function of metallicity, molecular gas opacity, line excitation, and other factors. Using spectral line observations of CO and its isotopologues mapped across a nearby galaxy, we can constrain the molecular gas conditions and link them to a variation in α_12CO(1−0). Here, we present new, wide-field (10 × 10 arcmin²) IRAM 30-m telescope 1 mm and 3 mm line observations of ¹²CO, ¹³CO, and C¹⁸O across the nearby, grand-design, spiral galaxy M101. From the CO isotopologue line ratio analysis alone, we find that selective nucleosynthesis and changes in the opacity are the main drivers of the variation in the line emission across the galaxy. In a further analysis step, we estimated α_{¹²CO(1−0)} using different approaches, including (i) via the dust mass surface density derived from far-IR emission as an independent tracer of the total gas surface density and (ii) local thermal equilibrium (LTE) based measurements using the optically thin ¹³CO(1–0) intensity. We find an average value of ⟨α_{¹²CO(1 − 0)}⟩ = 4.4  ±  0.9 M_⊙ pc⁻² (K km s⁻¹)⁻¹ across the disk of the galaxy, with a decrease by a factor of 10 toward the 2 kpc central region. In contrast, we find LTE-based α_{¹²CO(1−0)} values are lower by a factor of 2–3 across the disk relative to the dust-based result. Accounting for α_{¹²CO(1−0)} variations, we found significantly reduced molecular gas depletion time by a factor 10 in the galaxy’s center. In conclusion, our result suggests implications for commonly derived scaling relations, such as an underestimation of the slope of the Kennicutt Schmidt law, if α_{¹²CO(1−0)} variations are not accounted for.