The K-band Hubble diagram for the brightest cluster galaxies: a test of hierarchical galaxy formation models

ABSTRACT We analyse the K-band Hubble diagram for a sample of brightest cluster galax-ies (BCGs) in the redshift range 0 < z < 1. In good agreement with earlier stud-ies, we confirm that the scatter in the absolute magnitudes of the galaxies is small(0.3magnitudes). The BCGs exhibit very little luminosity evolution in this redshiftrange: if q 0 = 0.0 we detect no luminosity evolution; for q 0 = 0.5 we measure a smallnegative evolution (i.e., BCGs were about 0.5magnitudes fainter at z = 1 than to-day). If the mass in stars of these galaxies had remained constant over this periodof time, substantial positive luminosity evolution would be expected: BCGs shouldhave been brighter in the past since their stars were younger. A likely explanationfor the observed zero or negative evolution is that the stellar mass of the BCGs hasbeen assembled over time through merging and accretion, as expected in hierarchi-cal models of galaxy formation. The colour evolution of the BCGs is consistent withthat of an old stellar population (z