We report the discovery of JKCS 041 , a massive near-infrared selected cluster of galaxies at z _ { phot } \sim 1.9 .
The cluster was originally discovered using a modified red-sequence method and was 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 absence of the X-ray data .
We investigate the possibility that JKCS 041 is not a galaxy cluster at z \sim 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 ^ { +8.3 } _ { -7.6 } arcsec ( about 300 kpc ) , an X-ray temperature of 7.4 ^ { +5.3 } _ { -3.3 } keV , a bolometric X-ray luminosity within R _ { 500 } of ( 7.6 \pm 0.5 ) \times 10 ^ { 44 } erg s ^ { -1 } , and an estimated mass of M _ { 500 } = 2.9 ^ { +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 \pm 6.3 galaxies within 1.5 arcmin ( 750 kpc ) brighter than K \sim 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 } \sim 1.9 with a deep potential well , making it the most distant cluster with extended X-ray emission known .