The origin and evolution of the galaxy mass-metallicity relation

We use high-resolution cosmological zoom-in simulations from the Feedback in Realistic Environment (FIRE) project to study the galaxy mass-metallicity relations (MZR) from z = 0-6. These simulations include explicit models of the multiphase ISM, star formation, and stellar feedback. The simulations cover halo masses M-halo = 10(9)-10(13) M-circle dot and stellar masses M* = 10(4)-10(11) M-circle dot at z = 0 and have been shown to produce many observed galaxy properties from z = 0-6. For the first time, our simulations agree reasonably well with the observed mass-metallicity relations at z = 0-3 for a broad range of galaxy masses. We predict the evolution of the MZR from z = 0-6, as log(Z(gas)/Z(circle dot)) = 12 + log(O/H) -9.0 = 0.35[log(M*/M-circle dot) 10] + 0.93 exp(-0.43z) -1.05 and log(Z*/Z(circle dot)) = [Fe/H] + 0.2 = 0.40[log(M*/M-circle dot) 10] + 0.67 exp(-0.50z) -1.04, for gas-phase and stellar metallicity, respectively. Our simulations suggest that the evolution of MZR is associated with the evolution of stellar/gas mass fractions at different redshifts, indicating the existence of a universal metallicity relation between stellar mass, gas mass, and metallicities. In our simulations, galaxies above M* = 10(6)M(circle dot) are able to retain a large fraction of their metals inside the halo, because metal-rich winds fail to escape completely and are recycled into the galaxy. This resolves a longstanding discrepancy between 'subgrid' wind models (and semi-analytic models) and observations, where common subgrid models cannot simultaneously reproduce the MZR and the stellar mass functions.