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

The colour-magnitude relations (CMRs) of one of the most massive (~10¹⁵M_solar), 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 ~2700Å - 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.