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 controversy over the degree of BCG evolution is shown to be due to sample selection since the BCG luminosity depends on the cluster environment. Selecting only the most X-ray 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 models, and significant evidence of any growth only if the dominant stellar population of the BCGs formed relatively recently (z</=2.6).