The warps survey.: III.: The discovery of an X-ray luminous galaxy cluster at z=0.833 and the impact of X-ray substructure on cluster abundance measurements

The Wide Angle ROSAT Pointed Survey team reviews the properties and history of the discovery of Cl J0152.7 - 1357, an X-ray luminous, rich cluster of galaxies at a redshift of z = 0.833. At L-X = 8 x 10(44) h(50)(-2) ergs s(-1) (0.5-2.0 keV) Cl J0152.7-1357 is 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. This scenario is supported by the high degree of optical and X-ray substructure in Cl J0152.7-1357, which is similarly complex as that of other X-ray-selected clusters at comparable redshift and consistent with the hypothesized picture of cluster formation by mass infall along large-scale filaments. X-ray emission from Cl J0152.7-1357 was detected already in 1980 with the Einstein IPC. However, because the complex morphology of the emission caused its significance to be underestimated, the corresponding source was not included in the cluster sample of the Einstein Extended Medium-Sensitivity Survey (EMSS) and hence was not previously identified. Simulations of the EMSS source detection and selection procedure performed by us suggest a general, mild bias of the EMSS cluster sample against X-ray luminous clusters with pronounced substructure. If highly unrelaxed, merging clusters are common at intermediate-to-high redshift (as is suggested by the current data), they could create a bias in some samples as the morphological complexity of mergers may cause them to fall below the flux limit of surveys that make the implicit or explicit assumption of a unimodal spatial source geometry. Conversely, the enhanced X-ray luminosity of mergers might cause them to, temporarily, rise above the flux limit. Either effect could lead to erroneous conclusions about the evolution of the comoving cluster space density. A high fraction of morphologically complex clusters at high redshift would also call into question the validity of evolutionary studies (and, specifically, cosmological conclusions) which implicitly or explicitly assume that the systems under investigation are virialized.