Equilibrium Model Constraints on Baryon Cycling Across Cosmic Time
Galaxies strongly self-regulate their growth via energetic feedback from stars, supernovae, and black holes, but these processes are among the least understood aspects of galaxy formation theory. We present an analytic galaxy evolution model that directly constrains such feedback processes from observed galaxy scaling relations. The equilibrium model, which is broadly valid for star-forming central galaxies that dominate cosmic star formation, is based on the ansatz that galaxies live in a slowly-evolving equilibrium between inflows, outflows, and star formation. Using a Bayesian Monte Carlo Markov chain approach, we constrain our model to match observed galaxy scaling relations between stellar mass and halo mass, star formation rate, and metallicity from 0<z<2. A good fit (chi^2~1.6) is achieved with eight free parameters. We further show that constraining our model to any two of the three data sets also produces a fit to the third that is within reasonable systematic uncertainties. The resulting best-fit parameters that describe baryon cycling suggest galactic outflow scalings intermediate between energy and momentum-driven winds, a weak dependence of wind recycling time on mass, and a quenching mass scale that evolves modestly upwards with redshift. This model further predicts a stellar mass-star formation rate relation that is in good agreement with observations to z~6. Our results suggest that this simple analytic framework captures the basic physical processes required to model the mean evolution of stars and metals in galaxies, despite not incorporating many canonical ingredients of galaxy formation models such as merging or disk formation.