The metallicity of galactic winds
  The abundance evolution of galaxies depends critically on the balance between
the mixing of metals in their interstellar medium, the inflow of new gas and
the outflow of enriched gas. We study these processes in gas columns
perpendicular to a galactic disk using sub-parsec resolution simulations that
track stellar ejecta with the Flash code. We model a simplified interstellar
medium stirred and enriched by supernovae and their progenitors. We vary the
density distribution of the gas column and integrate our results over an
exponential disk to predict wind and ISM enrichment properties for disk
galaxies. We find that winds from more massive galaxies are hotter and more
highly enriched, in stark contrast to that which is often assumed in galaxy
formation models. We use these findings in a simple model of galactic
enrichment evolution, in which the metallicity of forming galaxies is the
result of accretion of nearly pristine gas and outflow of enriched gas along an
equilibrium sequence. We compare these predictions to the observed
mass-metallicity relation, and demonstrate how the galaxy's gas fraction is a
key controlling parameter. This explains the observed flattening of the
mass-metallicity relation at higher stellar masses.
