Scaling relations and baryonic cycling in local star-forming galaxies

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 star , from ∼10 7 − 10 11 M ⊙ . First, we find the metallicity ( Z ) dependence of the conversion factor for CO luminosity to molecular H 2 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 ∼0.2 dex; analogous relations for atomic gas mass M HI and total gas mass M gas are less accurate, ∼0.4 dex and ∼0.3 dex, 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 ∼0.05 dex. 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 ≲ 3 × 10 9 M ⊙ ) tend to be overwhelmed by (H I ) 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 (3 × 10 9 M ⊙ ≲ M star ≲3 × 10 10 M ⊙ ), star formation proceeds apace with gas availability, and H I and H 2 are both proportional to SFR. In the most massive “gas-poor, bimodality” regime ( M star ≳ 3 × 10 10 M ⊙ ), H I does not apparently participate in star formation, although it generally dominates in mass over H 2 . 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.

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