The intracluster gas fraction in X-ray clusters: constraints on the clustered mass density

Abstract

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 _gas(r₅₀₀) = (0.060 ± 0.003) h^−3/2. 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 Ω_m/Ω_b ≤ 23.1 h^3/2. Combining this with a maximal value of Ω_b from primordial nucleosynthesis results in Ω_mh^1/2 < 0.76. A more physically plausible approach based on low deuterium abundance inferences from quasar absorption spectra and accounting for baryons within cluster galaxies yields an estimate of Ω_mh^2/3 = 0.30 ± 0.07, with sources of systematic error involved in the derivation providing approximately 30 per cent additional uncertainty. Other effects which could enhance the likelihood of the Einstein-de Sitter case Ω_m = 1 are presented, and their observable signatures discussed.