2009A&A...507..147A

We report the discovery of JKCS041, a massive near-infrared selected cluster of galaxies at z_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 JKCS041 is secure, even in the absence of the X-ray data. We investigate the possibility that JKCS041 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 JKCS041 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^+8.3_–7.6arcsec (about 300kpc), an X-ray temperature of 7.4^+5.3_–3.3keV, a bolometric X-ray luminosity within R₅₀₀ of (7.6±0.5)x10⁴⁴erg/s, and an estimated mass of M₅₀₀=2.9^+3.8_–2.4x10¹⁴M_☉, the last derived under the usual (and strong) assumptions. The cluster is composed of 16.4±6.3 galaxies within 1.5arcmin (750kpc) brighter than K∼20.7mag. The high redshift of JKCS041 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, JKCS041 is a cluster of galaxies at z_phot∼1.9 with a deep potential well, making it the most distant cluster with extended X-ray emission known.