The colour-magnitude diagram ⋆

We present deep SofI and ISAAC near-infrared imaging data of the X-ray luminous galaxy cluster RDCS J1252.9- 2927. The ISAAC data were taken at the ESO Very Large Telescope under very good seeing conditions and reach limiting Vega magnitudes of 25.6 and 24.1 in the J− and Ks−bands respectively. The image quality is 0. '' 45 in both passbands. We use these data to construct a colour-magnitude (C-M) diagram of galaxies that are within 20 '' of the cluster center and brighter than Ks = 24, which is five magnitudes fainter than the apparent magnit ude of a L ⋆ galaxy in this cluster. The C-M relation is clearly identified as an over-density of galaxies with col ours near J− Ks = 1.85. The slope of the relation is−0.05± 0.02 and the intrinsic scatter is 0.06 magnitudes with a 90% confid ence interval that extends from 0.04 to 0.09 magnitudes. Both the slope and the scatter are consistent with the values measured for clusters at lower redshifts. These quantities have not evolved from z = 0 to z = 1.24. However, significant evolution in the mean J− Ks colour is detected. On average, the galaxies in RDCS J1252.9-2927 are 0.25 magnitudes bluer than early-type galaxies in the Coma cluster. Using instantaneous single-burst solar-metallicity models, the average age of galaxies in th e center of RDCS J1252.9-2927 is 2.7 Gyrs.