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 surv eys of clusters of galaxies is evidence in favor of hierarchical model s of the growth of structure in the universe. A large recent survey ha s, 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 hig h-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 cons istent with that of most previous surveys, but it occupies a flux rang e not previously covered (greater than 6 x 10(-14) ergs cm(-2) s(-1) t otal flux in the 0.5-2 keV band). At high redshifts (z > 0.3) the clus ter 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% confide nce) on the amplitude of any pure negative density evolution of cluste rs of these luminosities, in contrast with the factor of approximate t o 3 [corresponding to number density evolution proportional to (1 + z) (-2.5)] found in the Einstein Extended Medium-Sensitivity Survey at si milar 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 evol ution of the less luminous but more common optically poor clusters. Mo dels involving preheating of the X-ray gas at an early epoch fit the o bservations, at least for Omega(0) = 1.