Revisiting the Baryon Fractions of Galaxy Clusters: A Comparison with WMAP 3-year Results

The baryonic mass fraction (Ωb/Ωm) can be sensitively constrained using X-ray observations of galaxy clusters. In this paper, we compare the baryonic mass fraction inferred from measurements of the cosmic microwave background with the gas mass fractions (fgas) of a sample of 19 clusters taken from the recent literature. In systems cooler than 4 keV, fgas declines as the system temperature decreases. However, in higher temperature systems, fgas converges to (0.12± 0.02) (h/0.72) −1.5 , where the uncertainty reflects the systematic variations between clusters and the dependence on radius beyond ∼ r500. This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released WMAP 3-year results. We investigate possible reasons for this discrepancy, including the effects of radiative cooling and non-gravitational heating, and conclude that the most likely solution is that Ωm = 0.28–0.39, higher than the best-fit WMAP value, but consistent at the 2σ level. Degeneracies within the WMAP data require that σ8 must also be greater than the maximum likelihood value for consistency between the data sets.