The evolution of the red sequence slope in massive galaxy clusters

We investigate the evolution of the optical and near-infrared colour-magnitude relation in an homogeneous sample of massive clusters from z = 1 to the present epoch. By comparing deep Hubble Space Telescope Advanced Camera for Surveys imaging of X-ray selected MAssive Cluster Survey (MACS) clusters at z ~ 0.5 to the similarly selected Las Campanas/AAT Rich Cluster Survey (LARCS) sample at z ~ 0.1, we find that the rest-frame δ(U - V)/δV slope of the colour-magnitude relation evolves with redshift which we attribute to the build up of the red sequence over time. This rest-frame slope evolution is not adequately reproduced by that predicted from semi-analytic models based on the Millennium Simulation despite a prescription for the build up of the red sequence by in-falling galaxies, `strangulation'. We observe no strong correlation between this slope and the cluster environment at a given redshift demonstrating that the observed evolution is not due to a secondary correlation. Also presented are near-infrared United Kingdom Infrared Telescope (UKIRT) Wide Field CAMera (WFCAM) observations of the LARCS clusters which confirm and improve on the result from Stott et al. (2007) finding that there has been a two-fold increase in faint M_V > -20 galaxies on the red sequence since z = 0.5 to a significance of 5σ.