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

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 LX = 8×10 44 h 50 erg s −1 (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 (Ω0 > 0.5). This scenario is supported by the high degree of optical and X-ray substructure in ClJ0152.7–1357, 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. We re-assess the evidence for cluster evolution at high X-ray luminosities (LX ∼> 3× 10 44 h 50 erg s , 0.3− 3.5 keV) by comparing the cluster X-ray luminosity functions (XLF) at z < 0.3 (from the ROSAT Brightest Cluster Sample, BCS), 0.3 < z < 0.6 (from the revised EMSS sample of Ebeling & Henry, 1999), and 0.75 < z < 1 (from a joint EMSS-WARPS cluster sample). We find the differences between all three cumulative XLFs to be insignificant (less than 2σ) at essentially all luminosities, suggesting minimal evolution in the space density of X-ray luminous clusters out to z ∼ 1. If a moderate incompleteness of the EMSS of less than 15% is assumed at 0.3 < z < 0.6, the addition of three clusters of intermediate luminosity [LX = (7−11)×10 44 h 50 erg s −1 ] at 0.35 ≤ z ≤ 0.55 reduces the significance of any remaining negative evolution at z < 0.6 to less than 1σ at all X-ray luminosities. Similarly, the assumption that the EMSS or WARPS missed only one moderately X-ray luminous cluster at 0.75 < z < 1 erases the signature of negative evolution entirely also at high redshifts. Finally, we find a lower limit to the space density of very X-ray luminous clusters (LX ∼> 1.5 × 10 45 h 50 erg s , 0.3− 3.5 keV) at 0.75 < z < 1.5 implied by the detection of ClJ0152.7–1357 in the WARPS survey to be in excellent agreement with the local XLF from the BCS. Two cautionary notes are in order. Firstly, the statistical uncertainties of the XLFs discussed here do not allow us to rule out moderate negative evolution (up to a factor of 2.1 at LX ∼> 5× 10 44 h 50 erg s −1 out to z ∼ 0.6, and up to a factor of 5.3 at LX ∼> 1×10 45 h 50 erg s −1 out to z ∼ 1; both at 2σ confidence). Secondly, the tentative evidence for a high fraction of morphologically complex clusters at high redshift calls into question the validity of evolutionary studies (and, specifically, cosmological conclusions) that implicitly or explicitly assume that the systems under investigation are virialized. Deeper observations and more detailed analyses of a sizeable, representative sample of distant X-ray luminous clusters are required to conclusively address these issues. Subject headings: galaxies: clusters: general — galaxies: clusters: individual (ClJ0152.7–1357, MS1054.4–0321, MS1137.5+6625, RXJ1716.6+6708) — cosmology: observations — X-rays: general