HEAD Division Meeting 1999, April 1999
Session 13. Clusters and Galaxies
Oral, Tuesday, April 13, 1999, 8:30am-10:10am, Colonial Room

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[13.03] Cosmological constraints from the X-ray luminosity function of the ROSAT Brightest Cluster Sample

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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