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 ($R_e$) and lower velocity dispersions (sigma) at fixed
stellar mass M_star, and much larger R_e at a fixed $\sigma$ 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
towards larger R_e at fixed $\sigma$ compared to non-CGs with similar M_star.
The CG R_e-M_star relation also shows differences, primarily driven by a
sub-population (~15%) of galaxies with large $R_e$, while the M_star-sigma
relations are indistinguishable. These results are accentuated when double
Sersic 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 $R_e$ and M_star 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.
