The dependence of the galaxy mass-metallicity relation on environment
  and the implied metallicity of the IGM
  We explore the dependence of the galaxy mass-metallicity relation on
environment in SDSS, in terms of both over-density and central/satellite
dichotomy. We find that at a given stellar mass, there is a strong dependence
of metallicity on over-density for star-forming satellites. Instead, for
star-forming centrals no correlation is found. Star-forming satellites at
different stellar masses form a tight sequence in the average overdensity -
metallicity plane, which covers the entire observed range of metallicities and
stellar masses. This remarkable result appears to imply that there exists a
universal evolutionary path for all star-forming satellites, regardless of
their stellar masses. The strong correlation between over-density and
metallicity for star-forming satellites indicates that the gas inflow of
satellite galaxies is progressively metal-enriched in denser regions. We
interpret our results by employing the gas regulator model and find that the
metallicity of the enriched inflow of star-forming satellite galaxies strongly
increases with increasing over-density, largely independently of stellar mass.
If the metallicity of the inflow of star-forming satellites can represent the
metallicity of the IGM, then the implied metallicity of the IGM rises from ~
0.01 solar metallicity in void-like environments to ~ 0.3 solar metallicity in
cluster-like environments, in broad agreement with observations. We show that
the observed metallicity difference between star-forming centrals and
star-forming satellites becoming smaller towards high stellar masses can be
simply explained by the mass-independent enriched inflow, without the need to
involve any mass-dependent environmental effect on metallicity. Since satellite
galaxies account for at least half of the galaxy population, our findings
prompt for a revision of many galaxy evolutionary models, which generally
assume pristine gas inflows.
