Discovery of an X-Ray-luminous Galaxy Cluster at z=1.4

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^-1. 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)×10^-14 ergs cm^-2 s^-1, a rest-frame X-ray luminosity of L_X=(3.0+/-0.2)×10⁴⁴ h^-2₇₀ 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.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and NASA.

Based on observations obtained at the European Southern Observatory using the Very Large Telescope, on Cerro Paranal (ESO programs 72.A-0706, 73.A-0737, 74.A-0023, and 274.A-5024).