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 multi-phase ISM, star formation, and stellar feedback. The simulations cover halo masses M_(halo) = 10^9–10^(13) M_☉ and stellar masses M_* = 10^4–10^(11) M_☉ 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_☉) = 12+log(O/H)-9.0 = 0.35 [log(M_*/M_☉) - 10] + 0.93exp(-0.43z) - 1.05 and log(Z_*/Z_☉) = [Fe=H] + 0.2 = 0.40 [log(M_*/M_☉)-10]+0.67exp(-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_☉ 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 long-standing discrepancy between “sub-grid” wind models (and semi-analytic models) and observations, where common sub-grid models cannot simultaneously reproduce the MZR and the stellar mass functions