The colour–magnitude relations of ClJ1226.9+3332, a massive cluster of galaxies at z = 0.89

ABSTRACT The colour–magnitude relations (CMRs) of one of the most massive (≈10 15  M ⊙ ) , high‐redshift ( z = 0.89) clusters of galaxies known have been studied. Photometry has been measured in the V , R , I , z , F606W , F814W , J and K bands to a depth of K ≈ 20.5 = K *+ 2.5 and spectroscopy confirms 27 K ‐band selected galaxies as members of the cluster. The V − K colours are equivalent to a rest‐frame colour of ≈2700 Å− J , and provide a very sensitive measure of star formation activity. The Hubble Space Telescope imaging with the Advanced Camera for Surveys has been used to morphologically classify the galaxies. The cluster has a low early‐type fraction compared to nearby clusters, with only 33 per cent of the confirmed cluster members having types E or S0. The early‐type member galaxies form a clear red‐sequence in all colours. The scatter and slope of the relations show no evolution compared to the equivalent Coma cluster relations, suggesting the stellar populations are already very old at z = 0.89. The normalization of the relations has been compared to models based on synthetic stellar populations (SSPs), and are most consistent with stellar populations forming at z f ≳ 3 . Some galaxies of late‐type morphology were found to lie on the red‐sequence of the CMR, suggesting that they have very similar stellar populations to the early‐type galaxies. These results present a picture of a cluster in which the early‐type galaxies are all old, but in which there must be future morphological transformation of galaxies to match the early‐type fraction of nearby clusters. In order to preserve the tight CMR of early types seen in nearby clusters, the late‐type galaxies must transform their colours, through the cessation of star formation, before the morphological transformation occurs. Such evolution is observed in the late types lying on the CMR.