The evolution of Brightest Cluster Galaxies

In this contribution we review the properties of Brightest Cluster Galaxies (BCGs) and discuss the impact that X-ray cluster selection is having on their use as cosmological probes. BCGs form a unique galaxy population. They are located near the gravitational centre of galaxy clusters and are the most massive galaxies in the universe, being some 10 times more luminous than L* systems. Historically, BCGs have been credited with small intrinsic dispersion in their absolute magnitudes (DeltaMsimilar or equal to0.2-0.3 mag) and used as standard candles to constrain the cosmological parameters. Although indirect signs of mass accretion out to zsimilar or equal to1 have been observed, uncovering their full evolutionary picture has remained an elusive goal. Studies of BCGs based on serendipitiously discovered X-ray cluster samples, particularly from ROSAT, provide large numbers of unbiased clusters at zless than or equal to1. Furthermore X-ray emission guarantees the presence of a large gravitationally bound potential well and the X-ray information can be used to locate the centroids of clusters, aiding the identification of the BCG. We show that this has important consequences for studies of distance determination and large-scale streaming flows based on the optical properties of BCGs. Recent results based on X-ray selected clusters show large differences in near-IR BCG properties with their cluster environment; such that those in clusters with L(x)greater than or equal to1.9x10(44) erg s(-1) are brighter and more uniform than those in their low-L-x counterparts. The BCGs in high L-x systems show no evidence of having undergone mass growth, whereas those in low Lx systems show a wider range of evolution, with evidence that some have grown by a factor of 4 or more since zsimilar or equal to1. These results are a direct indication of how a single homogeneous population of galaxies evolves and are a challenge to simple semi-analytical hierarchical models. If future observations at high redshift are to seriously challenge theory then better predictions of the evolutionary process are required.