Red sequence determination of the redshift of the cluster of galaxies JKCS 041: z ∼ 2.2

This paper aims at robustly determining the redshift of the cluster of galaxies JKCS 041 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 JKCS 041. The JKCS 041 red sequence is 0.32+/-0.06 mag redder in z' - J than the red sequence of the z(spec) = 1.62 IRC0218A cluster, putting JKCS 041 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 JKCS 041, where the uncertainty accounts for uncertainties in stellar synthesis population models, in photometric calibration, and in the red sequence color of both JKCS 041 and IRC0218A clusters. We do not observe any sign of truncation of the red sequence for both clusters down to J = 23 mag (1 x 10(11) 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.