The WARPS survey. II. The log N log S relation and the X-ray evolution of low-luminosity clusters of galaxies

The strong negative evolution observed in previous X-ray-selected surveys of clusters of galaxies is evidence in favor of hierarchical models of the growth of structure in the universe. A large recent survey has, however, contradicted the low-redshift results, finding no evidence for evolution at z < 0.3. Here we present the first results from an X-ray-selected, flux-and surface brightness-limited deep survey for high-redshift clusters and groups of galaxies based on ROSAT PSPC pointed data. The log N-log S relation of all clusters in this survey is consistent with that of most previous surveys, but it occupies a flux range not previously covered (greater than 6 x 10(-14) ergs cm(-2) s(-1) total flux in the 0.5-2 keV band). At high redshifts (z > 0.3) the cluster luminosities are in the range 4 x 10(43)-2 x 10(44) h(50)(-2) ergs s(-1), the luminosities of poor clusters. The number of high-redshift, low-luminosity clusters is consistent with no evolution of the X-ray luminosity function between redshifts of z approximate to 0.4 and z = 0, and it places a limit of a factor of less than 1.7 (at 90% confidence) on the amplitude of any pure negative density evolution of clusters of these luminosities, in contrast with the factor of approximate to 3 [corresponding to number density evolution proportional to (1 + z)(-2.5)] found in the Einstein Extended Medium-Sensitivity Survey at similar redshifts but higher luminosities. Taken together, these results support hierarchical models in which there is mild negative evolution of the most luminous clusters at high redshift, but little or no evolution of the less luminous but more common optically poor clusters. Models involving preheating of the X-ray gas at an early epoch fit the observations, at least for Omega(0) = 1.