| Abstract
| - We report the discovery of JKCS 041, a massive near-infrared selected cluster of galaxies at $z_{\mathrm{phot}}$~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 ~ 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 $36.6^{\rm +8.3}_{-7.6}$ arcsec (about 300 kpc), an X-ray temperature of $7.4^{\rm +5.3}_{-3.3}$ keV, a bolometric X-ray luminosity within R500 of (7.6 ±0.5) $\times$ 10 44 erg s -1, and an estimated mass of M500 = $2.9^{\rm +3.8}_{-2.4}$ $\times$ 10 14$M_odot$, 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 ~ 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 $z_{\mathrm{phot}}$> 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 $z_{\mathrm{phot}}$ ~ 1.9 with a deep potential well, making it the most distant cluster with extended X-ray emission known.
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