A Comprehensive Study of the Outskirts of Galaxy Clusters Using Suzaku

Abstract

Galaxy clusters, which contain up to tens of thousands of galaxies and which are the largest virialized structures in the universe, serve as unique probes of cosmology. Most of their baryonic mass is in the form of hot gas that emits X-rays via thermal bremsstrahlung radiation. The study of this emission from the outer, least-relaxed portions of clusters yields valuable information about the hierarchical assembly of large scale structure. In this thesis, we report on our X-ray analysis of the outskirts of four clusters. For this purpose, we Suzaku data, which is well-suited to the study of the outsides of clusters. Accurate parameter estimates require reliable data and proper analysis, so we focus on the 0.7--7.0 keV range because other studies have shown that energies below or above this range are less reliable. A key component of our analysis is our careful modeling of the background emission as a thermal component plus a power law contribution. Our power law model uses a fixed slope of 1.4, which is consistent with other clusters. We constrain our thermal background component by fitting it to ROSAT data over the energy range 0.3--2.0~keV. Using this method, we extract the temperature, density, and surface brightness from the Suzaku data. These parameters are somewhat different from the values obtained using XMM-Newton data but are consistent with other measurements using Suzaku. We then deprojected these quantities to estimate the total mass, entropy, pressure, and baryonic fraction. We find an entropy that is consistent with the previously suggested universal' entropy profile, but our pressure deviates from the universal' profile. We discuss some possible reasons for this discrepancy. Consistent with previous observations but in contrast to what is expected from simulations, we infer that the outer parts of the clusters we study have baryon fractions in excess of the cosmic fraction. We suggest some explanations for this, focusing on clumping as a possibility. We then finish by discussing the role of our observations in cluster physics studies and by enumerating other avenues of exploration to attain a more complete picture of galaxy clusters.