Determination of the Baryon Fraction in Galaxy Clusters

An accurate estimate of the baryon fraction f_b in galaxy clusters plays a potentially important role in the `direct' measurement of the cosmic density parameter, Ω_m. In this talk, we first present the X-ray measurements of f_b among > 200 clusters under the conventional assumption that the intracluster gas is in isothermal and hydrostatic equilibrium with the underlying gravitational potential of clusters. Using a subsample of 80 clusters, we further examine the dependence of the baryon fraction upon the X-ray temperature and cluster radius, and compare the theoretically expected and observationally fitted X-ray luminosity (L,_x)-temperature (T) relation. While both the f_b-T and L_x,-T relations are consistent with the prediction of standard models that galaxy clusters of different temperatures share a common baryon fraction, the baryon fraction is a monotonically increasing function of cluster radius in the framework of isothermal β model and hydrostatic equilibrium for intracluster gas, and therefore its asymptotic trend at large radius cannot reach a universal value unless a mildly increasing (for 1/3 < β < 2/3) or decreasing (for β > 2/3) temperature profile with radius is invoked. Consequently, our current estimate of Ω_m based on f_b within a certain cutoff radius could contain large uncertainties.