Scaling relations and baryonic cycling in local star-forming galaxies. II. Gas content and star-formation efficiency

Abstract

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<SUB>star</SUB>, from ∼10<SUP>7</SUP> - 10<SUP>11</SUP> M<SUB>☉</SUB>. First, we find the metallicity (Z) dependence of the conversion factor for CO luminosity to molecular H<SUB>2</SUB> mass α<SUB>CO</SUB> to be shallower than previous estimates, with α<SUB>CO</SUB> ∝ (Z/Z<SUB>☉</SUB>)<SUP>-1.55</SUP>. Second, molecular gas mass M<SUB>H2</SUB> is found to be strongly correlated with M<SUB>star</SUB> and star-formation rate (SFR), enabling predictions of M<SUB>H2</SUB> good to within ∼0.2 dex; analogous relations for atomic gas mass M<SUB>HI</SUB> and total gas mass M<SUB>gas</SUB> are less accurate, ∼0.4 dex and ∼0.3 dex, respectively. Indeed, the behavior of atomic gas mass M<SUB>HI</SUB> 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<SUB>gas</SUB> is considered to depend on M<SUB>HI</SUB>, together with M<SUB>star</SUB> and SFR, we obtain a relation that predicts M<SUB>gas</SUB> to within ∼0.05 dex. Finally, the analysis of depletion times and the scaling of M<SUB>HI</SUB>/M<SUB>star</SUB> and M<SUB>H2</SUB>/M<SUB>star</SUB> 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<SUB>star</SUB> ≲ 3 × 10<SUP>9</SUP> M<SUB>☉</SUB>) tend to be overwhelmed by (H I) accretion, and despite short τ<SUB>H2</SUB> (and thus presumably high star-formation efficiency), star formation is unable to keep up with the gas supply. For galaxies in the intermediate M<SUB>star</SUB> "gas-equilibrium" bin (3 × 10<SUP>9</SUP> M<SUB>☉</SUB> ≲ M<SUB>star</SUB> ≲3 × 10<SUP>10</SUP> M<SUB>☉</SUB>), star formation proceeds apace with gas availability, and H I and H<SUB>2</SUB> are both proportional to SFR. In the most massive "gas-poor, bimodality" regime (M<SUB>star</SUB> ≳ 3 × 10<SUP>10</SUP> M<SUB>☉</SUB>), H I does not apparently participate in star formation, although it generally dominates in mass over H<SUB>2</SUB>. 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.