Molecular gas scaling relations for local star-forming galaxies in the low-M_* regime

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern, 0315 Oslo, Norway
e-mail: bendix.hagedorn@astro.uio.no
² Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
³ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁵ Dipartimento di Fisica e Astronomia “Augusto Righi”, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁶ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy

Received: 28 February 2024
Accepted: 6 May 2024

Abstract

We derived molecular gas fractions (f_mol = M_mol/M_*) and depletion times (τ_mol = M_mol/SFR) for 353 galaxies representative of the local star-forming population with 10^8.5 M_⊙ < M_* < 10^10.5 M_⊙ drawn from the ALLSMOG and xCOLDGASS surveys of CO(2−1) and CO(1−0) line emission. By adding constraints from low-mass galaxies and upper limits for CO non-detections, we find the median molecular gas fraction of the local star-forming population to be constant at log f_mol = −0.99_−0.19^+0.22, challenging previous reports of increased molecular gas fractions in low-mass galaxies. Above M_* ∼ 10^10.5 M_⊙, we find the f_mol versus M_* relation to be sensitive to the selection criteria for star-forming galaxies. We tested the robustness of our results against different prescriptions for the CO-to-H₂ conversion factor and different selection criteria for star-forming galaxies. The depletion timescale τ_mol weakly depends on M_*, following a power law with a best-fit slope of 0.16 ± 0.03. This suggests that small variations in specific star formation rate (sSFR = SFR/M_*) across the local main sequence of star-forming galaxies with M_* < 10^10.5 M_⊙ are mainly driven by differences in the efficiency of converting the available molecular gas into stars. We tested these results against a possible dependence of f_mol and τ_mol on the surrounding (group) environment of the targets by splitting them into centrals, satellites, and isolated galaxies, and find no significant variation between these populations. We conclude that the group environment is unlikely to have a large systematic effect on the molecular gas content of star-forming galaxies in the local Universe.