The universal baryonic mass fraction ( \Omega _ { b } / \Omega _ { 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 } ( r _ { 500 } ) converges to \approx \left ( 0.12 \pm 0.02 \right ) \left ( h / 0.72 \right ) ^ { -1.5 } , where the uncertainty reflects the systematic variations between clusters at r _ { 500 } .
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 \Omega _ { m } is higher than the best-fit WMAP value ( we find \Omega _ { m } = 0.36 ^ { +0.11 } _ { -0.08 } ) , but consistent at the 2 \sigma level .
Degeneracies within the WMAP data require that \sigma _ { 8 } must also be greater than the maximum likelihood value for consistency between the data sets .