Understanding the unique assembly history of central group galaxies

Central galaxies (CGs) in massive halos live in unique environments with formation histories closely linked to that of the host halo. In local clusters, they have larger sizes (Re ) and lower velocity dispersions (σ) at fixed stellar mass M *, and much larger Re at a fixed σ than field and satellite galaxies (non-CGs). Using spectroscopic observations of group galaxies selected from the COSMOS survey, we compare the dynamical scaling relations of early-type CGs and non-CGs at z ~ 0.6 to distinguish possible mechanisms that produce the required evolution. CGs are systematically offset toward larger Re at fixed σ compared to non-CGs with similar M *. The CG Re -M * relation also shows differences, primarily driven by a subpopulation (~15%) of galaxies with large Re , while the M *-σ relations are indistinguishable. These results are accentuated when double Sérsic profiles, which better fit light in the outer regions of galaxies, are adopted. They suggest that even group-scale CGs can develop extended components by these redshifts that can increase total Re and M * estimates by factors of ~2. To probe the evolutionary link between our sample and cluster CGs, we also analyze two cluster samples at z ~ 0.6 and z ~ 0. We find similar results for the more massive halos at comparable z, but much more distinct CG scaling relations at low-z. Thus, the rapid, late-time accretion of outer components, perhaps via the stripping and accretion of satellites, would appear to be a key feature that distinguishes the evolutionary history of CGs.