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H. Ebeling (IfA), A.C. Fabian, S.W. Allen (Inst. of Astronomy), A.C. Edge (Univ. of Durham), C.S. Crawford (Inst. of Astronomy)
The ROSAT Brightest Cluster Sample (BCS) is the largest X-ray selected and X-ray flux limited cluster sample compiled to date. In 1997 we modelled the X-ray luminosity function (XLF) of the 197 BCS clusters with fluxes higher than 4.4 \times 10-12 erg s-1 cm-2 (0.1-2.4 keV) and redshifts less than z=0.3 with a simple Schechter function model and found no signs of significant evolution out to z\approx 0.2. Using an extended sample of 300 clusters with fluxes higher than 2.8 \times 10-12 erg s-1 cm-2 we now investigate the cosmological implications of this lack of evolution by fitting to our data a more physical XLF model based on the Press-Schechter mass function. Thanks to the, by X-ray cluster standards, large size of our sample we are able to constrain simultaneously all three fit parameters \Omega0, \sigma8, and n (the density parameter, the dispersion of the primordial mass fluctuation spectrum on the scale of 8 Mpc, and the power law slope of the mass power spectrum on cluster scales). Our best fit value of \Omega0=0.37+0.11-0.10 (1\sigma) is consistent with other recent measurements and adding to the growing evidence in support of a low-density universe. A non-zero cosmological constant is not ruled out by our data which are equally well fit by a flat world model with an only slightly higher value of \Omega0.
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