2011A&A...526A..11A

This paper aims at robustly determining the redshift of the cluster of galaxies JKCS041 and at putting constraints on the formation epoch of the color-magnitude sequence in two very high redshift clusters. New deep z'-J data show a clear narrow red sequence that is co-centered with, and similary concentrated on, the extended X-ray emission of the cluster of galaxies JKCS041. The JKCS041 red sequence is 0.32±0.06mag redder in z'-J than the red sequence of the z_spec=1.62 IRC0218A cluster, putting JKCS041 at z≫1.62 and ruling out z≤1.49 the latter claimed by a recent paper. The color difference of the two red sequences gives a red-sequence-based redshift of z=2.20±0.11 for JKCS041, where the uncertainty accounts for uncertainties in stellar synthesis population models, in photometric calibration, and in the red sequence color of both JKCS041 and IRC0218A clusters. We do not observe any sign of truncation of the red sequence for both clusters down to J=23mag (1x10¹¹ solar masses), which suggests that it is already in place in clusters rich and massive enough to heat and retain hot gas at these high redshifts.