The metallicity of galactic winds

The abundance evolution of galaxies depends critically on the balance between the mixing of metals in their interstellar medium (ISM), the inflow of new gas and the outflow of enriched gas. We study these processes in gas columns perpendicular to a galactic disc using sub-parsec resolution simulations that track stellar ejecta with the FLASH code. We model a simplified ISM stirred and enriched by supernovae and their progenitors. We vary the density distribution of the gas column and integrate our results over an exponential disc to predict wind and ISM enrichment properties for disc 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.