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_s-bands respectively. The image quality is 0.45 arcsec in both passbands. We use these data to construct a colour-magnitude (C-M) diagram of galaxies that are within 20 arcsec of the cluster center and brighter than K_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_s=1.85. The slope of the relation is -0.05 ± 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_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.

Based on observations obtained at the European Southern Observatory using the ESO Very Large Telescope on Cerro Paranal (ESO program 166.A-0701(B)) and the ESO New Technology Telescope on Cerro La Silla (ESO program 61.A-0676(A)).