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, early-type brightest cluster galaxies (BCGs) in the Sloan Digital Sky Survey (SDSS) MaxBCG and C4 catalogues have larger sizes than early-type galaxies of similar luminosity, whether these other objects are in the field or are satellites in clusters. BCG sizes are also stronger functions of luminosity R e α L than are the sizes of the bulk of the population; this remains true if one restricts attention to narrow bins in velocity dispersion. At fixed stellar mass and formation time, objects at lower redshift are larger and have smaller velocity dispersions - i.e. the sizes increase and velocity dispersions decrease with age. In addition, at any given redshift, younger BCGs have slightly larger sizes than older BCGs of the same stellar mass; however, they have similar velocity dispersions. As a result, at redshifts ∼0.25, corresponding to lookback times of order 3 Gyr, BCGs are smaller than their lower redshift counterparts by as much as ∼70 per cent for the brightest BCGs: the sizes evolve 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 M r -22 the evolution is approximately (1 + z) ―0.7 , independent of M r . The velocity dispersion-luminosity correlation also evolves: (1 + z) ―0.2(Mr+21) at M r < ―22 (as for the BCGs) and (1 + z) 0.2 for fainter galaxies. The size- and velocity dispersion-stellar mass correlations yield consistent results, although, in this case, accounting for selection effects is less straightforward. These trends, in particular the fact that the velocity dispersions at fixed stellar mass decrease with age, 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 super-dense galaxies seen at z ∼ 2; major dry mergers, which increase the size in proportion to the mass, cannot bring these galaxies on to the BCG R e ― M * relation at z ∼ 0. We also compared the ages and sizes of our early-type BCGs with other cluster galaxies (satellites). BCGs are larger than satellites of similar luminosity or stellar mass 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 mag more luminous. Finally, we find that the mean satellite luminosity is approximately independent of BCG luminosity, in agreement with recent predictions based on the luminosity dependence of clustering.