The WARPS survey: III. The discovery of an X-ray luminous galaxy cluster at z=0.833 and the evolution of X-ray luminous clusters at z<1

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Abstract
The Wide Angle ROSAT Pointed Survey (WARPS) reports the discovery of ClJ0152.7-1357, an X-ray luminous, rich cluster of galaxies at a redshift of z=0.833. At L_X = 8 x 10^44 erg/s (0.5-2.0 keV) ClJ0152.7-1357 is, together with MS1054.4-0321, the most X-ray luminous cluster known at redshifts z>0.55. The high X-ray luminosity of the system suggests that massive clusters may begin to form at redshifts considerably greater than unity which is difficult to reconcile with a high value of the density parameter of the Universe. We re-assess the evidence for cluster evolution at high X-ray luminosities by comparing the cluster X-ray luminosity functions (XLF) at z<0.3 (from the BCS), 0.3<z<0.6 (from the revised EMSS sample of Ebeling & Henry, 1999), and 0.75<z 1.5 x 10^45 erg/s, 0.3-3.5 keV) at 0.75<z 5 x 10^44 erg/s and z 1 x 10^45 erg/s and z<1) is not ruled out by our data. And secondly, the tentative evidence for a high fraction of morphologically complex clusters at high redshift calls into question the validitly of evolutionary studies that assume clusters to be virialized.

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