GAS REGULATION OF GALAXIES: THE EVOLUTION OF THE COSMIC SPECIFIC STAR FORMATION RATE, THE METALLICITY–MASS–STAR-FORMATION RATE RELATION, AND THE STELLAR CONTENT OF HALOS

A very simple physical model of galaxies is one in which the formation of stars is instantaneously regulated by the mass of gas in a reservoir with mass loss scaling with the star-formation rate (SFR). This model links together three different aspects of the evolving galaxy population: (1) the cosmic time evolution of the specific star-formation rate (sSFR) relative to the growth of halos, (2) the gas-phase metallicities across the galaxy population and over cosmic time, and (3) the ratio of the stellar to dark matter mass of halos. The gas regulator is defined by the gas consumption timescale (ε−1) and the mass loading λ of the wind outflow λ·SFR. The simplest regulator, in which ε and λ are constant, sets the sSFR equal to exactly the specific accretion rate of the galaxy; more realistic situations lead to an sSFR that is perturbed from this precise relation. Because the gas consumption timescale is shorter than the timescale on which the system evolves, the metallicity Z is set primarily by the instantaneous operation of the regulator system rather than by the past history of the system. The metallicity of the gas reservoir depends on ε, λ, and sSFR, and the regulator system therefore naturally produces a Z(mstar, SFR) relation if ε and λ depend on the stellar mass mstar. Furthermore, this relation will be the same at all epochs unless the parameters ε and λ themselves change with time. A so-called fundamental metallicity relation is naturally produced by these conditions. The overall mass–metallicity relation Z(mstar) directly provides the fraction fstar(mstar) of incoming baryons that are being transformed into stars. The observed Z(mstar) relation of Sloan Digital Sky Survey (SDSS) galaxies implies a strong dependence of stellar mass on halo mass that reconciles the different faint-end slopes of the stellar and halo mass functions in standard ΛCDM models. The observed relation also boosts the sSFR relative to the specific accretion rate and produces a different dependence on mass, both of which are observed. The derived Z(mstar, SFR) relation for the regulator system is fit to published Z(mstar, SFR) data for the SDSS galaxy population, yielding ε and λ as functions of mstar. The fitted ε is consistent with observed molecular gas-depletion timescales in galaxies (allowing for the extra atomic gas), while the fitted λ is also reasonable. The gas-regulator model also successfully reproduces the Z(mstar) metallicities of star-forming galaxies at z ∼ 2. One consequence of this analysis is that it suggests that the mstar–mhalo relation is established by baryonic processes operating within galaxies, and that a significant fraction (40%) of baryons coming into the halos are being processed through the galaxies. This fraction may be more or less constant. The success of the gas-regulator model in simultaneously explaining many diverse observed relations over the 0 < z < 2 interval suggests that the evolution of galaxies is governed by simple physics that form the basis for this model.