Recycled stellar ejecta as fuel for star formation and implications for the origin of the galaxy mass-metallicity relation

TL;DR

This study uses cosmological simulations to quantify how recycled stellar ejecta fuel star formation, revealing its increasing importance over time and its role in shaping galaxy metallicity and the mass-metallicity relation.

Contribution

It provides the first detailed quantification of recycled stellar ejecta's contribution to star formation and galaxy metallicity across different galaxy masses and environments.

Findings

Recycled stellar ejecta contribute up to 35% of the cosmic star formation rate at z=0.

Recycling's importance increases with galaxy mass below 10^{10.5} M_sun.

Recycling correlates positively with galaxy metallicity and influences the mass-metallicity relation.

Abstract

We use cosmological, hydrodynamical simulations from the EAGLE and OWLS projects to assess the significance of recycled stellar ejecta as fuel for star formation. The fractional contributions of stellar mass loss to the cosmic star formation rate (SFR) and stellar mass densities increase with time, reaching $35 \%$ and $19 \%$, respectively, at $z=0$. The importance of recycling increases steeply with galaxy stellar mass for $M_{\ast} < 10^{10.5}$ M$_{\odot}$, and decreases mildly at higher mass. This trend arises from the mass dependence of feedback associated with star formation and AGN, which preferentially suppresses star formation fuelled by recycling. Recycling is more important for satellites than centrals and its contribution decreases with galactocentric radius. The relative contribution of AGB stars increases with time and towards galaxy centers. This is a consequence of the more gradual release of AGB ejecta compared to that of massive stars, and the preferential removal of the latter by star formation-driven outflows and by lock up in stellar remnants. Recycling-fuelled star formation exhibits a tight, positive correlation with galaxy metallicity, with a secondary dependence on the relative abundance of alpha elements (which are predominantly synthesized in massive stars), that is insensitive to the subgrid models for feedback. Hence, our conclusions are directly relevant for the origin of the mass-metallicity relation and metallicity gradients. Applying the relation between recycling and metallicity to the observed mass-metallicity relation yields our best estimate of the mass-dependent contribution of recycling. For centrals with a mass similar to that of the Milky Way, we infer the contributions of recycled stellar ejecta to the SFR and stellar mass to be $35 \%$ and $20 \%$, respectively.