How Environment Affects Galaxy Metallicity through Stripping and
  Formation History: Lessons from the Illustris Simulation
  Recent studies have 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 directly affecting metallicity.
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 our 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 on 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, however, is caused by the higher concentration of
star-forming disks of satellite galaxies, as this biases their SFR-weighted
metallicities toward 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, the metallicities of satellites
and centrals become environmentally independent once they are 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.
