THE INSIDE-OUT GROWTH OF THE MOST MASSIVE GALAXIES AT 0.3 <z< 0.9

Abstract

We study the surface brightness profiles of a sample of brightest cluster galaxies (BCGs) with 0.3<z<0.9. The BCGs are selected from the first Red-sequence Cluster Survey and an X-ray cluster survey. The surface brightness profiles of the BCGs are measured using HST ACS images, and most of them can be well modeled by a Sersic profile with index ~ 6 and half-light radius ~ 30 kpc. Although the single Sersic model fits the profiles well, we argue that the systematics in the sky background measurement and the coupling between the model parameters make the comparison of the model parameters ambiguous. Direct comparison of the BCG profiles, on the other hand, has revealed an inside-out growth for these most massive galaxies: as the mass of a BCG increases, the central mass density of the galaxy increases slowly ($\rho_{1kpc} \propto M*^{0.39}$), while the slope of the outer profile grows continuously shallower ($\alpha \propto M*^{-2.5}$). Such a fashion of growth continues down to the less massive early-type galaxies (ETGs), without apparent distinction between BCGs and non-BCGs. For the very massive ETGs and BCGs, the slope of the Kormendy relation starts to trace the slope of the surface brightness profiles and becomes insensitive to subtle profile evolution. These results are generally consistent with dry mergers being the major driver of the mass growth for BCGs and massive ETGs. We also find strong correlations between the richness of clusters and the properties of BCGs: the more massive the clusters are, the more massive the BCGs ($M*_{bcg} \propto M_{clusters}^{0.6}$) and the shallower their surface brightness profiles. After taking into account the bias in the cluster samples, we find the masses of the BCGs have grown by at least a factor of 1.5 from z=0.5 to z=0, in contrast to the previous findings of no evolution. Such an evolution validates the expectation from the LCDM model.