PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: Physical drivers of the molecular gas fraction, R_mol

¹ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁴ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
⁵ Center for Cosmology and Astroparticle Physics, 191 West Woodruff Avenue, Columbus, OH 43210, USA
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁷ Department of Physics, University of Alberta, Edmonton AB T6G 2E1, Canada
⁸ Department of Astronomy, University of Cape Town, Private Bag X3, 7701 Rondebosch, South Africa
⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁰ Netherlands Institute for Radio Astronomy (ASTRON), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹¹ European Southern Observatory, Karl-Schwarzschild Straße 2, D-85748 Garching bei München, Germany
¹² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹³ Institute of Astronomy and Astrophysics, Academia Sinica, Astronomy-Mathematics Building, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
¹⁴ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA
¹⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching, Germany
¹⁶ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Straße 2, 69120 Heidelberg, Germany
¹⁷ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁸ Department of Physics, University of Connecticut, Storrs, CT 06269, USA
¹⁹ Department of Physics and Astronomy, McMaster University, 1280 Main St. W., Hamilton, ON. L8S 4L8, Canada
²⁰ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, E-28014 Madrid, Spain
²¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, D-69120 Heidelberg, Germany
²² Department of Physics, Tamkang University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan
²³ Center for Astrophysics and Space Sciences, Department of Physics, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
²⁴ Sub-department of Astrophysics, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

^⋆ Corresponding author; ceibenst@nrao.edu

Received: 11 March 2024
Accepted: 1 July 2024

Abstract

The molecular-to-atomic gas ratio is crucial to our understanding of the evolution of the interstellar medium (ISM) in galaxies. We investigated the balance between the atomic (Σ_HI) and molecular gas (Σ_H2) surface densities in eight nearby star-forming galaxies using new high-quality observations from MeerKAT and ALMA (for H I and CO, respectively). We defined the molecular gas ratio as R_mol = Σ_H2/Σ_HI and measured how R_mol depends on local conditions in the galaxy disks using multiwavelength observations. We find that, depending on the galaxy, H I is detected at > 3σ out to 20 − 120 kpc in galactocentric radius (r_gal). The typical radius at which Σ_HI reaches 1 M_⊙ pc⁻² is r_H I ≈ 22 kpc, which corresponds to 1 − 3 times the optical radius (r₂₅). We note that, R_mol correlates best with the dynamical equilibrium pressure, P_DE, among potential drivers studied, with a median correlation coefficient of ⟨ρ⟩ = 0.89. Correlations between R_mol and the star formation rate surface density, total gas surface density, stellar surface density, metallicity, and Σ_SFR/P_DE (a proxy for the combined effect of the UV radiation field and number density) are present but somewhat weaker. Our results also show a direct correlation between P_DE and Σ_SFR, supporting self-regulation models. Quantitatively, we measured similar scalings as previous works, and attribute the modest differences that we do find to the effect of varying resolution and sensitivity. At r_gal ≳ 0.4r₂₅, atomic gas dominates over molecular gas among our studied galaxies, and at the balance of these two gas phases (R_mol = 1), we find that the baryon mass is dominated by stars, with Σ_* > 5 Σ_gas. Our study constitutes an important step in the statistical investigation of how local galaxy properties (stellar mass, star formation rate, or morphology) impact the conversion from atomic to molecular gas in nearby galaxies.