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 (2.4 sigma) 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 (f) over bar(gas)(r(500)) = (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 Omega(m)/Omega(b) less than or equal to 23.1 h(3/2). Combining this with a maximal value of Omega(b), from primordial nucleosynthesis results in Omega(b) h(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 Omega(m) h(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 Omega(m) = 1 are presented, and their observable signatures discussed.