Properties of the Intracluster Medium in an Ensemble of Nearby Galaxy Clusters

Abstract

We present a systematic analysis of the intracluster medium (ICM) in an X-ray 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 β model profiles, and extract ICM central densities and radial distributions. We use the data and an ensemble of numerical cluster simulations to quantify sources of uncertainty for all reported parameters. We examine the ensemble properties within the context of models of structure formation and feedback from galactic winds. We present best-fit ICM mass-temperature M ICM -⟨ T X ⟩ relations for M ICM calculated within r 500 and 1 h -1 50 Mpc. These relations exhibit small scatter (17%), providing evidence of regularity in large, X-ray flux limited cluster ensembles. Interestingly, the slope of the M ICM -⟨ T X ⟩ relation (at limiting radius r 500 ) is steeper than the self-similar expectation by 4.3 σ. 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 that of the hotter clusters ⟨ f ICM ⟩=0.212 ± 0.006 (90% confidence intervals). In apparent contradiction with previously published analyses, our large, X-ray flux limited cluster sample provides no evidence for a more extended radial ICM distribution in low-⟨ T X ⟩ clusters down to the sample limit of 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 overestimated by ~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 provides a firm upper limit on the cosmological density parameter for clustered matter Ω M ≤(0.36 ± 0.01) h -½ 50 .