How Environment Affects Galaxy Metallicity through Stripping and Formation History: Lessons from the Illustris Simulation

Shy Genel

Published 2016-02-08Version 1

Recent studies found higher galaxy metallicities in richer environments. It is not yet clear, however, whether metallicity-environment dependencies are merely an indirect consequence of environmentally-dependent formation histories, or of environment-related processes affecting metallicity directly. Here we present a first detailed study of metallicity-environment correlations in a cosmological hydrodynamical simulation, in particular the Illustris simulation. Illustris galaxies display similar relations to those observed. Utilizing knowledge of simulated formation histories, and leveraging the large simulation volume, we construct galaxy samples of satellites and centrals that are matched in formation histories. This allows us to find that ~1/3 of the metallicity-environment correlation is due to different formation histories in different environments. This is a combined effect of satellites (in particular in denser environments) having average lower z=0 star-formation rates (SFRs), and of their older stellar ages even at a given z=0 SFR. Most of the difference, ~2/3, is however caused by the higher concentration of the star-forming disks of satellite galaxies, since it biases their SFR-weighted metallicities towards their inner, more metal-rich, parts. With a newly defined quantity, the 'radially-averaged' metallicity, which captures the metallicity profile but is independent of the SFR profile, metallicities of satellites and centrals become environmentally independent, once matched in formation history. We find that circumgalactic metallicity (defined as rapidly inflowing gas around the virial radius), while sensitive to environment, has no measurable effect on the metallicity of the star-forming gas inside the galaxies.