The Origin and Evolution of the Galaxy Mass–Metallicity Relation

The Origin and Evolution of the Galaxy Mass–Metallicity Relation

Abstract

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 $M_{\text{halo}} = 10^9 - 10^{13} M_⊙$ 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

$\log(Z_{gas}/Z_{\odot}) = 12+\log(O/H)−9.0 = 0.35[\log(M_{*}/M_{\odot}) −10] + 0.93 \exp(−0.43z) − 1.05$

and

$\log(Z_{*} / Z_{\odot}) = [Fe/H] + 0.2 = 40[\log(M_{*}/M_{\odot}) - 10] + 0.67 \exp(-0.50z) - 1.04$

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 $M_{*} = 10^6 M_{\odot}$ 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.