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

TL;DR

This study uses cosmological simulations to investigate the origins and evolution of the galaxy mass-metallicity relationship, highlighting the roles of gas outflows and star formation efficiency over cosmic time.

Contribution

It demonstrates that low star formation efficiency, regulated by supernova feedback, is the main driver of the mass-metallicity trend in galaxies, with minimal evolution in shape but increasing normalization.

Findings

Simulated galaxies match observed MZR at z=0 and z=2.

Baryon loss occurs due to UV heating in low-mass galaxies.

Supernova feedback regulates star formation and metallicity.

Abstract

We examine the origin and evolution of the mass-metallicity relationship (MZR, M-Z) for galaxies using high resolution cosmological SPH + N-Body simulations that include a physically motivated description of supernovae feedback and subsequent metal enrichment. We discriminate between two sources that may contribute to the origin of the MZR: 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 MZR in shape and normalization both at z=0 and z=2. We find that baryon loss occurs due to UV heating before star formation turns on in galaxies with M_baryon < 10^8 M_sun, but that some gas loss due to supernovae induced winds is required to subsequently reproduce the low effective chemical yield observed in low mass galaxies. Despite this, 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 MZR 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. 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.