Evolution of brightest cluster galaxies in X-ray clusters

A recent paper presents the analysis of the K-band Hubble diagram of 76 brightest cluster galaxies (BCGs) in X-ray clusters and shows that the properties of BCGs depend on the X-ray luminosity (L-X) of their host clusters. Unfortunately, the low numbers of nearby clusters in this sample makes it difficult to constrain evolutionary trends. In this letter we extend the Hubble diagram of Burke, Collins & Mann to a total of 155 clusters using new data on 79 BCGs at z less than or equal to 0.1 from the 2MASS extended source catalogue. We show that the major division between BCGs in high- and low-L-X clusters disappears at z less than or equal to 0.1, with BCGs having similar absolute magnitudes independent of the X-ray luminosity of their host clusters. At larger redshifts, the K-band light of BCGs in high-L-X systems is consistent with little or no merging back to z similar to 0.8, whereas BCGs in the low-L-X systems have a different evolutionary history, with many increasing their mass by a factor greater than or equal to4 since z similar or equal to 1. This provides direct evidence of hierarchical merging in a galaxy population.