Assembly Conformity of Structure Growth: Fossil versus Normal Groups of Galaxies

Abstract

Using a semianalytic method calibrated to the global star formation history and the stellar mass function at z  = 0, we attempt to understand the most stellar-deficient galaxy groups. We argue that such groups are a kind of fossil group (FGs)—in comparison to the normal groups of galaxies, they assemble both halo and stellar mass earlier. We find that there is a central galaxy and satellite conformity between these FGs and normal groups: centrals and satellites in the former form earlier and are more stellar deficient than the counterparts of the latter. We term this effect “assembly conformity” of dark matter halos. This effect accounts for about 70% of the difference in stellar content between FGs and normal groups. When split by the peak redshift for the star formation rate of a group, the mass functions of satellite halos on either side of the peak redshift are found to be indistinguishable between FGs and normal groups, indicating a self-similarity of halo assembly with respect to the peak. The “baryonic environmental” effect due to ram pressure and gas heating accounts for about 30% of the difference in stellar content. While the total stellar mass of FGs is lower than that of normal groups, we predict that the mass of the brightest central galaxy of FGs is, on average, higher than that of normal groups. We also predict that in the central galaxies of FGs, there is a negative stellar age gradient from the center outward, where the opposite is expected for those in normal groups.