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This paper aims at robustly determining the redshift of the cluster ofgalaxies JKCS041 and at putting constraints on the formation epoch of thecolor-magnitude sequence in two very high redshift clusters. New deep z'-J datashow a clear narrow red sequence that is co-centered with, and similarlyconcentrated on, the extended X-ray emission of the cluster of galaxiesJKCS041. The JKCS041 red sequence is 0.32+/-0.06 mag redder in z'-J than thered sequence of the zspec=1.62 IRC0218A cluster, putting JKCS041 at z>>1.62 andruling out z<~1.49 the latter claimed by a recent paper. The color differenceof the two red sequences gives a red-sequence-based redshift of z=2.20+/-0.11for JKCS041, where the uncertainty accounts for uncertainties in stellarsynthesis population models, in photometric calibration, and in the redsequence color of both JKCS041 and IRC0218A clusters. We do not observe anysign of truncation of the red sequence for both clusters down to J=23 mag(1.0e+11 solar masses), which suggests that it is already in place in clustersrich and massive enough to heat and retain hot gas at these high redshifts.

Red sequence determination of the redshift of the cluster of galaxies JKCS041: z~2.2