The Origin and Evolution of the Mass-metallicity Relationship for Galaxies: Results from Cosmological N-body Simulations

We examine the origin and evolution of the mass-metallicity relationship (M∗-Z) for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of the effects of supernovae feedback and subsequent metal enrichment. Our simulations allow us to distinguish between two possible sources that contribute to both the origin of the mass-metallicity relationship and to the low chemical yield observed at low galaxy masses: 1) metal and baryon loss due to gas outflow, or 2) inefficient star formation at the lowest galaxy masses. Our simulated galaxies reproduce the observed M∗-Z relationship in shape and normalization both at z=0 and z=2. We show that low star formation efficiencies, regulated by supernovae feedback, are primarily responsible for the lower metallicities of low mass galaxies and the overall M∗-Z trend. We find that the shape of the M∗-Z relation is relatively constant with redshift, but that its normalization increases with time. Simulations with no energy feedback from supernovae overproduce metals at low galaxy masses by rapidly transforming a large fraction of their gas into stars. We find that gas mass loss due to supernovae induced winds and the cosmic UV field becomes significant in our galaxies with Mbar < 10 8 M⊙. Some gas loss due to supernovae feedback is necessary to reproduce the observed low effective yield observed in low mass galaxies. Despite the fact that our low mass galaxies have lost a majority of their baryons, they are still the most gas rich objects in our simulations due to their low star formation efficiencies. Subject headings: galaxies: evolution — galaxies: formation — methods: N-Body simulations