2007MNRAS.377.1457M

The universal baryonic mass fraction (Ω_b/Ω_m) 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 (f_gas) of a large sample of clusters taken from the recent literature. In systems cooler than 4 keV, f_gas declines as the system temperature decreases. However, in higher temperature systems, f_gas(r₅₀₀) converges to ~(0.12±0.02)(h/0.72)^–1.5, where the uncertainty reflects the systematic variations between clusters at r₅₀₀. This is significantly lower than the maximum-likelihood value of the baryon fraction from the recently released Wilkinson Microwave Anisotropy Probe (WMAP) 3-yr 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 Ω_m is higher than the best-fitting WMAP value (we find Ω_m= 0.36^+0.11_–0.08), 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.