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 (i.e. all galaxies members of groups/clusters which are not centrals). High metallicity satellites reside, on average, in regions four times denser than the low metallicity ones. Instead, for star-forming centrals no correlation is found. Star-forming satellites at different stellar masses form a tight sequence in the average over-density - 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, Z_{0, sat}, strongly increases with increasing over-density. The derived Z_{0, sat}- overdensity relation is largely independent of stellar mass and can be well described by a simple power law. 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 Z_⊙ in the void-like environment to ∼0.3 Z_⊙ in the cluster-like environment, 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.