Evolution in the structural properties of early-type Brightest Cluster Galaxies at small lookback time and dependence on the environment

Abstract

At the present time, SDSS early-type BCGs have larger Re than early-type galaxies of similar L, whether these other objects are in the field, or are satellites in clusters (Re ~ L for BCGs). At fixed M* and formation time, BCGs at lower z are larger and have smaller velocity dispersions, i.e. Re increases and sigma decreases with age. As a result, at z~0.25, corresponding to a lookback time of order 3 Gyrs, BCGs are smaller than their lower z counterparts by as much as 70% for the brightest BCGs: Re evolves as (1+z)^{0.85(Mr+21)}. Qualitatively similar but weaker evolution in the sizes is also seen in the bulk of the early-type population: at Mr<-22 Re evolves as (1+z)^{0.7(Mr+21)}, while at Mr>-22 the evolution is approximately (1+z)^{-0.7}, independent of Mr. The sigma-L correlation also evolves: (1+z)^{-0.2(Mr+21)} at Mr < -22 (as for the BCGs) and (1+z)^{0.2} for fainter galaxies. The Re- and sigma-M* correlations yield consistent results. These trends are most easily understood if early-type BCGs grew from many dry minor mergers rather than a few major mergers. Only in such a scenario can BCGs be the descendents of the superdense galaxies seen at z~2; major dry mergers, which increase the size in proportion to the mass, cannot bring these galaxies onto the BCG Re-M* relation at z~0. We also compared the ages and sizes of our early-type BCGs with satellites. BCGs are larger than satellites of similar L or M* at the same redshift. Although both satellites and BCGs trace the same weak age-L or age-M* relation, this can be understood by noting that BCGs are typically about 1 Gyr older than the satellites in their group, and they are about 0.5 mags more luminous. Finally, we find that the mean satellite L is approximately independent of BCG L, in agreement with recent predictions based on the L-dependence of clustering.