JKCS 041: a colour-detected galaxy cluster at zphot ∼ 1.9 with deep potential well as confirmed by X-ray data

We report the discovery of JKCS 041, a massive near-infrared selected cluster of galaxies at z(phot) similar to 1.9. The cluster was originally discovered using a modified red-sequence method and also detected in follow-up Chandra data as an extended X-ray source. Optical and near- infrared imaging data alone allow us to show that the detection of JKCS 041 is secure, even in the absence of the X-ray data. We investigate the possibility that JKCS 041 is not a galaxy cluster at z similar to 1.9, and find other explanations unlikely. The X-ray detection and statistical arguments rule out the hypothesis that JKCS 041 is actually a blend of groups along the line of sight, and we find that the X-ray emitting gas is too hot and dense to be a filament projected along the line of sight. The absence of a central radio source and the extent and morphology of the X-ray emission argue against the possibility that the X-ray emission comes from inverse Compton scattering of CMB photons by a radio plasma. The cluster has an X-ray core radius of 36.6(-7.6)(+8.3) arcsec (about 300 kpc), an X-ray temperature of 7.4(-3.3)(+5.3) keV, a bolometric X-ray luminosity within R-500 of (7.6 +/- 0.5) x 10(44) erg s(-1), and an estimated mass of M-500 = 2.9(-2.4)(+3.8) x 10(14) M-circle dot, the last derived under the usual (and strong) assumptions. The cluster is composed of 16.4 +/- 6.3 galaxies within 1.5 arcmin (750 kpc) brighter than K similar to 20.7 mag. The high redshift of JKCS 041 is determined from the detection colour, from the detection of the cluster in a galaxy sample formed by z(phot) > 1.6 galaxies and from a photometric redshift based on 11-band spectral energy distribution fitting. By means of the latter we find the cluster redshift to be 1.84 < z < 2.12 at 68% confidence. Therefore, JKCS 041 is a cluster of galaxies at z(phot) similar to 1.9 with a deep potential well, making it the most distant cluster with extended X-ray emission known.