The Mass-Metallicity Relation in Cosmological Hydrodynamic Simulations

Abstract

We use cosmological hydrodynamic simulations with enriched galacticoutflows to compare predictions for the galaxy mass-metallicity (M_* − Z)with observations at z ≈ 2 from Erb et al. (2006). With no outflowsincluded galaxies are over-enriched, indicating that outflows are requirednot only to suppress star formation and enrich the IGM but also to lowergalaxy metal content. The observed M_* − Z slope is matched both inour model without winds as well as in our favored outflow model where theoutflow velocity scales as the escape velocity, but is too steep ina model with constant outflow speeds. If outflows are too widespreadat early times, the IGM out of which smaller galaxies form can becomepre-polluted, resulting in a low-mass flattening of the M_* − Z relationthat is inconsistent with data. Remarkably, the same momentum-drivenwind model that provides the best agreement with IGM enrichment dataalso yields the best agreement with the z ≈ 2 M_* − Z relation,showing the proper outflow scaling and strength to match the observedslope and amplitude. In this model, the M_* − Z relation evolves slowlyfrom z = 6 → 2; an (admittedly uncertain) extrapolation to z = 0broadly matches local M_* − Z observations. Overall, the M_* − Zrelation provides critical constraints on galactic outflow processesduring the heyday of star formation in the Universe.