The metal enrichment of passive galaxies in cosmological simulations of galaxy formation

TL;DR

This paper demonstrates that cosmological simulations with AGN feedback can reproduce observed metallicity and alpha-element trends in massive galaxies, highlighting challenges in simultaneously matching multiple galaxy properties.

Contribution

It shows that quenching by radio-mode AGN feedback naturally reproduces observed alpha/Fe and metallicity relations in massive galaxies.

Findings

Simulations with AGN feedback match observed alpha/Fe vs. velocity dispersion.

Early quenching prevents massive galaxies from reacquiring enriched gas.

Reproducing both alpha/Fe and metallicity relations simultaneously remains challenging.

Abstract

Massive early-type galaxies have higher metallicities and higher ratios of $\alpha$ elements to iron than their less massive counterparts. Reproducing these correlations has long been a problem for hierarchical galaxy formation theory, both in semi-analytic models and cosmological hydrodynamic simulations. We show that a simulation in which gas cooling in massive dark haloes is quenched by radio-mode active galactic nuclei (AGNs) feedback naturally reproduces the observed trend between $\alpha$/Fe and the velocity dispersion of galaxies, $\sigma$. The quenching occurs earlier for more massive galaxies. Consequently, these galaxies complete their star formation before $\alpha$/Fe is diluted by the contribution from type Ia supernovae. For galaxies more massive than $\sim 10^{11}~M_\odot$ whose $\alpha$/Fe correlates positively with stellar mass, we find an inversely correlated mass-metallicity relation. This is a common problem in simulations in which star formation in massive galaxies is quenched either by quasar- or radio-mode AGN feedback. The early suppression of gas cooling in progenitors of massive galaxies prevents them from recapturing enriched gas ejected as winds. Simultaneously reproducing the [$\alpha$/Fe]-$\sigma$ relation and the mass-metallicity relation is, thus, difficult in the current framework of galaxy formation.