JKCS 041 : a colour – detected galaxy cluster at z = 1 . 9 with deep potential well as confirmed by X - ray data

We report the discovery of JKCS041, a massive near-infrared selected cluster of galaxies z = 1.9. The cluster was originally discovered using a modified red-sequence method and was also detected in follow-up Chandra data as extended X-ray source. Optical and near-infrared imaging data alone allow us to show that the detection of JKCS041 is as secure as the detections of clusters in the REFLEX survey. JKCS041 is also detected using an SED fitting technique, based on photometry in eleven bands. We investigate the possibility that JKCS041 is not a discrete galaxy cluster at z = 1.9, and find other explanations to be 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 is due to inverse Compton scattering of CMB photons by a radio plasma. The cluster has an X-ray core radius of 36.6 −7.6 arcsec (about 300 kpc), an X-ray temperature of 7.6 −3.3 keV, a bolometric X-ray luminosity within R500 of (7.6±0.5)×10 44 erg s, an estimated mass of M500 = 2.9 +3.8 −2.4 × 10 M⊙, the latter 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 JKCS041 is determined from the colour of the red sequence, from the detection of the cluster in a galaxy sample formed by zphot > 1.6 galaxies, and from SED fitting with z = 1.9 red galaxies. Therefore, JKCS041 is a cluster of galaxies at z = 1.9 with deep potential well, making it the most distant cluster with extended X-ray emission known.