2024A&A...687A.244H

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_{\mathrm{mol}}=-0.99^{+0.22}_{-0.19} $, 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.