Galaxies spatially coincident with the JKCS041 X-ray emission display a red sequence at z~2.2

Abstract: New deep z'-J data readly show a narrow red sequence co-centered with, and similary concentrated to, 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. The colour 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 colour of both JKCS041 and IRC0218A clusters.