Revisiting the Baryon Fractions of Galaxy Clusters: A Comparison with WMAP 3-year Results
Abstract
The universal baryonic mass fraction (Omegab/Omegam) 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 large sample of 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(r500) converges to approx. (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 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 Omegam is higher than the best-fit WMAP value (we find Omegam = 0.36+0.11-0.08), but consistent at the 2-sigma level. Degeneracies within the WMAP data require that sigma8 must also be greater than the maximum likelihood value for consistency between the data sets.
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