2020A&A...643A.180H

Assessments of the cold-gas reservoir in galaxies are a cornerstone for understanding star-formation processes and the role of feedback and baryonic cycling in galaxy evolution. Here we exploit a sample of 392 galaxies (dubbed MAGMA, Metallicity and Gas for Mass Assembly), presented in a recent paper, to quantify molecular and atomic gas properties across a broad range in stellar mass, M_star, from ∼10⁷-10¹¹M_☉. First, we find the metallicity (Z) dependence of the conversion factor for CO luminosity to molecular H₂ mass α_CO to be shallower than previous estimates, with α_CO ∝ (Z/Z_☉)^–1.55. Second, molecular gas mass M_H2 is found to be strongly correlated with M_star and star-formation rate (SFR), enabling predictions of M_H2 good to within ∼0.2dex; analogous relations for atomic gas mass M_HI and total gas mass M_gas are less accurate, ∼0.4dex and ∼0.3dex, respectively.Indeed, the behavior of atomic gas mass M_HI in MAGMA scaling relations suggests that it may be a third, independent variable that encapsulates information about the circumgalactic environment and gas accretion.If M_gas is considered to depend on M_HI, together with M_star and SFR, we obtain a relation that predicts M_gas to within ∼0.05dex. Finally, the analysis of depletion times and the scaling of M_HI/M_star and M_H2/M_star over three different mass bins suggests that the partition of gas and the regulation of star formation through gas content depends on the mass regime. Dwarf galaxies (M_star≤3x10⁹M_☉) tend to be overwhelmed by (HI) accretion, and despite short τ_H2 (and thus presumably high star-formation efficiency), star formation is unable to keep up with the gas supply. For galaxies in the intermediate M_star "gas-equilibrium" bin (3x10⁹M_☉≤M_star ≤3 x 10¹⁰M_☉), star formation proceeds apace with gas availability, and HI and H₂ are both proportional to SFR.In the most massive "gas-poor, bimodality" regime (M_star≥3x10¹⁰M_☉), HI does not apparently participate in star formation, although it generally dominates in mass over H₂. Our results confirm that atomic gas plays a key role in baryonic cycling, and is a fundamental ingredient for current and future star formation, especially in dwarf galaxies.