An Analytic Model for the Evolution of the Stellar, Gas, and Metal
  Content of Galaxies
  We present an analytic formalism that describes the evolution of the stellar,
gas, and metal content of galaxies. It is based on the idea, inspired by
hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium
between inflow, outflow, and star formation. We argue that this formalism
broadly captures the behavior of galaxy properties evolving in simulations. The
resulting equilibrium equations for the star formation rate, gas fraction, and
metallicity depend on three key free parameters that represent ejective
feedback, preventive feedback, and re-accretion of ejected material. We
schematically describe how these parameters are constrained by models and
observations. Galaxies perturbed off the equilibrium relations owing to inflow
stochasticity tend to be driven back towards equilibrium, such that deviations
in star formation rate at a given mass are correlated with gas fraction and
anti-correlated with metallicity. After an early gas accumulation epoch,
quiescently star-forming galaxies are expected to be in equilibrium over most
of cosmic time. The equilibrium model provides a simple intuitive framework for
understanding the cosmic evolution of galaxy properties, and centrally features
the cycle of baryons between galaxies and surrounding gas as the driver of
galaxy growth.
