The Intracluster Gas Fraction in X–ray Clusters : Constraints on the Clustered Mass Density

The mean intracluster gas fraction of X–ray clusters within their hydrostatic regions is derived from recent observational compilations of David, Jones & Forman and White & Fabian. At radii encompassing a mean density 500 times the critical value, the individual sample bi-weight means are moderately (2.4σ) discrepant; revising binding masses with a virial relation calibrated by numerical simulations removes the discrepancy and results in a combined sample mean and standard error f̄gas(r500) = (0.060± 0.003)h . For hierarchical clustering models with an extreme physical assumption to maximize cluster gas content, this value constrains the universal ratio of total, clustered to baryonic mass Ωm/Ωb ≤ 23.1 h ; combining with the primordial nucleosynthesis upper limit on Ωb results in Ωm h 1/2 < 0.60. A less conservative, physically plausible approach based on low D/H inferences from quasar absorption spectra and accounting for baryons within cluster galaxies yields an estimate Ωm h 2/3 = 0.28± 0.07 with sources of systematic error involved in the derivation providing approximately 35% uncertainty. Additional effects which could provide consistency with the Einstein– deSitter case Ωm=1 are presented, and their observable signatures discussed.