Properties of the intracluster medium in an ensemble of nearby galaxy clusters

We present a systematic analysis of the intracluster medium (ICM) in an X-r ay flux limited sample of 45 galaxy clusters. Using archival ROSAT Position -Sensitive Proportional Counter (PSPC) data and published ICM temperatures, we present best-fit double and single beta model profiles, and extract ICM central densities and radial distributions. We use the data and an ensembl e of numerical cluster simulations to quantify sources of uncertainty for a ll reported parameters. We examine the ensemble properties within the conte xt of models of structure formation and feedback from galactic winds. We pr esent best-fit ICM mass-temperature M-ICM-[T-X] relations for M-ICM calcula ted within r(500) and 1 h(50)(-1) Mpc. These relations exhibit small scatte r (17%), providing evidence of regularity in large, X-ray flux limited clus ter ensembles. Interestingly, the slope of the M-ICM-[T-X] relation (at lim iting radius r(500)) is steeper than the self-similar expectation by 4.3 si gma. We show that there is a mild dependence of ICM mass fraction f(ICM) on [T-X]; the clusters with ICM temperatures below 5 keV have a mean ICM mass fraction [f(ICM)] = 0.160 +/- 0.008, which is significantly lower than tha t of the hotter clusters [f(ICM)] = 0.212 +/- 0.006 (90% confidence interva ls). In apparent contradiction with previously published analyses, our larg e, X-ray flux limited cluster sample provides no evidence for a more extend ed radial ICM distribution in low-[T-X] clusters down to the sample limit o f 2.4 keV. By analyzing simulated clusters we find that density variations enhance the cluster X-ray emission and cause M-ICM and f(ICM) to be overest imated by similar to 12%. Additionally, we use the simulations to estimate an f(ICM) depletion factor at r(500). We use the bias corrected mean f(ICM) within the hotter cluster subsample as a lower limit on the cluster baryon fraction. In combination with nucleosynthesis constraints this measure pro vides a firm upper limit on the cosmological density parameter for clustere d matter Omega(M) less than or equal to (0.36 +/- 0.01) h(50)(-1/2).