2001ApJ...548L..23E

We report the discovery of the galaxy cluster Cl J1226.9+3332 in the Wide Angle ROSAT Pointed Survey (WARPS). At z=0.888 and L_X=1.1x10⁴⁵ h^–2₅₀ ergs.s^–1 (0.5-2.0 keV), Cl J1226.9+3332 is the most distant X-ray luminous cluster currently known. The mere existence of this system represents a huge problem for Ω₀=1 world models.

At the modest (off-axis) resolution of the ROSAT Position Sensitive Proportional Counter observation in which the system was detected, Cl J1226.9+3332 appears relaxed; an off-axis High Resolution Imager observation confirms this impression and rules out significant contamination from point sources. However, in moderately deep optical images (R and I band), the cluster exhibits signs of substructure in its apparent galaxy distribution. A first crude estimate of the velocity dispersion of the cluster galaxies based on six redshifts yields a high value of 1650 km.s^–1, indicative of a very massive cluster and/or the presence of substructure along the line of sight. While a more accurate assessment of the dynamical state of this system requires much better data at both optical and X-ray wavelengths, the high mass of the cluster has already been unambiguously confirmed by a very strong detection of the Sunyaev-Zeldovich effect in its direction.

Using Cl J1226.9+3332 and Cl J0152.7-1357 (z=0.835), the second most distant X-ray luminous cluster currently known and also a WARPS discovery, we obtain a first estimate of the cluster X-ray luminosity function at 0.8<z<1.4 and L_X>5x10⁴⁴ h^–2₅₀ ergs.s^–1 (0.5-2.0 keV). Using the best currently available data, we find the comoving space density of very distant, massive clusters to be in excellent agreement with the value measured locally (z<0.3) and conclude that negative evolution is not required at these luminosities out to z∼1. Our findings are in conflict with earlier claims of highly significant (>3 σ) negative evolution already at 0.3<z<0.6 based on the cluster samples of the Einstein Extended Medium-Sensitivity Survey (EMSS) and the Center for Astrophysics 160° survey. Our results agree, however, with the lack of significant evolution of very X-ray luminous clusters out to z∼0.4 reported by the MAssive Cluster Survey team. Our findings are also consistent with the abundance of very X-ray luminous clusters at z∼0.8 inferred from the EMSS cluster sample, provided Cl J0152.7-1357 (which was missed by the EMSS) is added in.