Cluster selection and the evolution of brightest cluster galaxies

The K-band Hubble diagram of brightest cluster galaxies (BCGs) is presented for a large, X-ray-selected cluster sample extending out to z = 0.8. The c ontroversy over the degree of BCG evolution is shown to be due to sample se lection since the BCG luminosity depends on the cluster environment. Select ing only the most Xray luminous clusters produces a BCG sample that shows, under the assumption of an Einstein-de Sitter cosmology, significantly less mass growth than that predicted by current semianalytic galaxy formation m odels, and significant evidence of any growth only if the dominant stellar population of the BCGs formed relatively recently (z less than or equal to 2.6).