2005ApJ...623L..85M

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 overdensity 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≃2.1 identifies it as a high-redshift candidate. Subsequent FORS2 multiobject 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±265 km/s. VLT ISAAC K_s- 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 that the cluster has an aperture-corrected unabsorbed X-ray flux of f_X=(3.6±0.3)x10^–14 ergs/cm2/s, a rest-frame X-ray luminosity of L_X=(3.0±0.2)x10⁴⁴ h^–2₇₀ergs/s (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.