ROSAT PSPC observations of 36 high-luminosity clusters of galaxies: constraints on the gas fraction

Abstract

We present a detailed and homogeneous analysis of the ROSAT PSPC surface brightness profiles of 36 clusters of galaxies with high X-ray luminosity (L_X > 10^{45} erg s^{-1}) and redshifts between 0.05 and 0.44. Using recent ASCA estimates of the temperature of the gas for most of the clusters in the sample, we apply both the deprojection technique and model fitting to the surface brightness profiles to constrain the gas and dark matter distributions under the assumption that the gas is both isothermal and hydrostatic. Applying robust estimators, we find that the gas fraction within r_{500} of the clusters in our sample has a distribution centred on f_gas(r_{500}) = 0.168 h_{50}^{-1.5}. The gas fraction ranges from 0.101 to 0.245 at the 95 per cent confidence level. The values of f_gas show highly significant variations between individual clusters, which may be explained if the dark matter has a significant baryonic component. Within a cluster, the average radial dependence of the gas mass fraction increases outward as r^s, with s~0.20. Combining these results with those of primordial nucleosynthesis calculations and the current estimate of H_0, the above central location implies \Omega_{0, m} < 0.56 at the 95 per cent confidence level. This upper limit decreases to 0.34 if we take the highest significant estimates for f_gas. A significant decrease in cluster gas fraction with redshift from the local value, f_{gas, 0}, of 0.21, found assuming \Omega_{0, m} =1, is also reduced if \Omega_{0, m} is low.