UNDERSTANDING THE UNIQUE ASSEMBLY HISTORY OF CENTRAL GROUP GALAXIES

Abstract

Central Galaxies (CGs) in massive halos live in unique environments with\nformation histories closely linked to that of the host halo. In local clusters\nthey have larger sizes ($R_e$) and lower velocity dispersions (sigma) at fixed\nstellar mass M_star, and much larger R_e at a fixed $\\sigma$ than field and\nsatellite galaxies (non-CGs). Using spectroscopic observations of group\ngalaxies selected from the COSMOS survey, we compare the dynamical scaling\nrelations of early-type CGs and non-CGs at z~0.6, to distinguish possible\nmechanisms that produce the required evolution. CGs are systematically offset\ntowards larger R_e at fixed $\\sigma$ compared to non-CGs with similar M_star.\nThe CG R_e-M_star relation also shows differences, primarily driven by a\nsub-population (~15%) of galaxies with large $R_e$, while the M_star-sigma\nrelations are indistinguishable. These results are accentuated when double\nSersic profiles, which better fit light in the outer regions of galaxies, are\nadopted. They suggest that even group-scale CGs can develop extended components\nby these redshifts that can increase total $R_e$ and M_star estimates by\nfactors of ~2. To probe the evolutionary link between our sample and cluster\nCGs, we also analyze two cluster samples at z~0.6 and z~0. We find similar\nresults for the more massive halos at comparable z, but much more distinct CG\nscaling relations at low-z. Thus, the rapid, late-time accretion of outer\ncomponents, perhaps via the stripping and accretion of satellites, would appear\nto be a key feature that distinguishes the evolutionary history of CGs.\n