Red sequence determination of the redshift of the cluster of galaxies
  JKCS041: z~2.2
  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 similarly
concentrated on, the extended X-ray emission of the cluster of galaxies
JKCS041. The JKCS041 red sequence is 0.32+/-0.06 mag redder in z'-J than the
red sequence of the zspec=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=23 mag
(1.0e+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.
