A new local temperature distribution function for X-ray clusters: cosmological applications

We present a new determination of the local temperature function of X-ray c lusters using a sample of X-ray clusters with fluxes above 2.2 10(-11) erg/ s/cm(2) in the [0.1 - 2.4] keV band, most of these clusters come from the A bell XBAC's sample to which a handful of known non-Abell clusters has been added. We estimate this sample to be 85% complete, and should therefore pro vide a useful estimation of the present-day number density of clusters. Com prising fifty clusters for which the temperature information is available, it is the largest complete sample of this kind. It is therefore expected to significantly improve the estimation of the temperature distribution funct ion of clusters. We find that the resulting temperature function is higher than previous estimations, but it agrees with the temperature distribution function inferred from the BCS and PASS luminosity function (Ebeling et al. 1997; De Grandi et al. 1999a). We have used this sample to constrain the a mplitude of the matter fluctuations a, on cluster's scale of 8(3)root Omega (-1)(0)h(-1)Mpc, assuming a mass-temperature relation based on recent nume rical simulations. We find sigma (c) similar to 0.6 +/- 0.02 for an Omega ( 0) = 1 model (for which sigma (c) = sigma (8)). Our sample provides a usefu l reference at z similar to 0 to use in the application of the cosmological test based on the evolution of X-ray clusters abundance (Oukbir & Blanchar d 1992, 1997). We have therefore estimated the temperature distribution fun ction at z = 0.33 using Henry's sample of high-z X-ray clusters (Henry 1997 ; hereafter H97) and performed a preliminary estimate of Omega (0). We find that the abundance of clusters at z = 0.33 is significantly smaller, by a factor larger than 2, which shows that the EMSS sample provides strong evid ence for evolution of the cluster abundance. A likelihood analysis leads to a rather high value of the mean density parameter of the universe: Omega ( 0) = 0.92(-0.215)(+0.255) (1 sigma) for open universes and Omega (0) = 0.86 5(-0.245)(+0.35) for flat universes, which is consistent with a previous in dependent estimation based on the full EMSS sample by Sadat et al. (1998). Some systematic uncertainties which could alter this result are briefly dis cussed.