Deep near-infrared imaging of RDCS J1252.9–2927 at z = 1.237 - 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 $K_{\mathrm s}-$bands respectively. The image quality is 0 $\farcs$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 $K_{\mathrm s}=24$, which is five magnitudes fainter than the apparent magnitude of a $L^{\star}$ galaxy in this cluster. The C-M relation is clearly identified as an over-density of galaxies with colours near $J-K_{\mathrm s}=1.85$. The slope of the relation is $-0.05 \pm 0.02$ and the intrinsic scatter is 0.06 mag with a 90% confidence interval that extends from 0.04 to 0.09 mag. 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 $J-K_{\mathrm s}$ colour is detected. On average, the galaxies in RDCS J1252.9-2927 are 0.25 mag bluer than early-type galaxies in the Coma cluster. Using instantaneous single-burst solar-metallicity models, the average age of galaxies in the center of RDCS J1252.9-2927 is 2.7 Gyrs.