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 clusters using a sample of X-ray clusters with fluxes above 2.2 10−11erg/s/cm2 in the [0.1 − 2.4] keV band, most of these clusters come from the Abell 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 provide a useful estimation of the present-day number density of clusters. Comprising 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 function 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 RASS luminosity function (Ebeling et al. 1997; De Grandi et al. 1999a). We have used this sample to constrain the amplitude of the matter fluctuations σc on cluster’s scale of 8 3 √ Ω0−1h−1Mpc, assuming a mass-temperature relation based on recent numerical simulations. We find σc ∼ 0.6± 0.02 for an Ω0 = 1 model (for which σc = σ8). Our sample provides a useful reference at z ∼ 0 to use in the application of the cosmological test based on the evolution of X-ray clusters abundance (Oukbir & Blanchard 1992, 1997). We have therefore estimated the temperature distribution function at z = 0.33 using Henry’s sample of high-z X-ray clusters (Henry 1997; hereafter H97) and performed a preliminary estimate of Ω0. 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 evidence for evolution of the cluster abundance. A likelihood analysis leads to a rather high value of the mean density parameter of the universe: Ω0 = 0.92 −0.215 (1σ) for open universes and Ω0 = 0.865 −0.245 for flat universes, which is consistent with a previous independent estimation based on the full EMSS sample by Sadat et al. (1998). Some systematic uncertainties which could alter this result are briefly discussed.