We report the discovery of a massive , X-ray-luminous cluster of galaxies at z =1.393 , the most distant X-ray–selected cluster found to date .
XMMU J2235.3–2557 was serendipitously detected as an extended X-ray source in an archival XMM-Newton observation of NGC 7314 .
VLT-FORS2 R and z band snapshot imaging reveals an over-density of red galaxies in both angular and color spaces .
The galaxy enhancement is coincident in the sky with the X-ray emission ; the cluster red sequence at R - z \simeq 2.1 identifies it as a high-redshift candidate .
Subsequent VLT-FORS2 multi-object spectroscopy unambiguously confirms the presence of a massive cluster based on 12 concordant redshifts in the interval 1.38 < z < 1.40 .
The preliminary cluster velocity dispersion is 762 \pm 265 km s ^ { -1 } .
VLT-ISAAC Ks and J band images underscore the rich distribution of red galaxies associated with the cluster .
Based on a 45 ks XMM-Newton observation , we find the cluster has an aperture-corrected , unabsorbed X-ray flux of f _ { X } = ( 3.6 \pm 0.3 ) \times 10 ^ { -14 } ergs cm ^ { -2 } s ^ { -1 } , a rest-frame X-ray luminosity of L _ { X } = ( 3.0 \pm 0.2 ) \times 10 ^ { 44 } ~ { } h _ { 70 } ^ { -2 } ergs s ^ { -1 } ( 0.5–2.0 keV ) , and a temperature of kT = 6.0 ^ { +2.5 } _ { -1.8 } keV .
Though XMMU J2235.3–2557 is likely the first confirmed z > 1 cluster found with XMM-Newton , the relative ease and efficiency of discovery demonstrates that it should be possible to build large samples of z > 1 clusters through the joint use of X-ray and large , ground-based telescopes .