X-Ray Studies of Galaxy Clusters and Groups

Abstract

The history and evolution of the universe, the understanding of the formation of structures and the investigation of a wealth of astrophysical phenomena are the topical subjects in present-day galaxy cluster physics and cosmology. Galaxy clusters are important laboratories as they are the largest distinct building blocks in the universe and embedded in the cosmological large-scale structure. In common concepts, clusters and groups evolved from tiny overdensities in the early universe and grow at the node points of the filaments through the merging of substructures and accretion of material. They are thus direct probes of structure formation theories and the cosmological standard model as well as essential objects to study astrophysical imprints of these formation processes in the hot intracluster medium (ICM) which is the largest baryonic component in clusters. X-ray observations are excellent tools to analyze the ICM properties in great detail which is the main focus of this work. <br /> In the first project, which was published in <em>Astronomy & Astrophysics</em> (Thölken et al., 2016), the gas properties of the galaxy group UGC03957 are investigated out to very large radii (1.4 <em>R</em>₂₀₀) with the Suzaku satellite. Due to the low surface brightness of groups and instrumental limitations, the outskirts of these objects are much less studied than galaxy clusters outskirts. However, Suzaku is able to reach these faint outer regions that are potentially influenced by structure formation effects. Previous analyses of cluster outskirts revealed interesting astrophysical effects such as gas clumping and non-equilibrium states. Here, the temperature, metal abundance, entropy and gas mass fraction profiles of the ICM are investigated and the following results are obtained. The metal abundance profile suggests a primary ICM enrichment by galactic winds and the abundance pattern yields a relative enrichment contribution for core-collapse supernovae of 80% – 100%. The temperature drops by a factor of three from the center to the outskirts which is consistent with findings for galaxy clusters. The latter often show a drop or flattening of the entropy profile at large radii and in some cases an excess in the gas mass fraction profile compared to the cosmic mean. Such an entropy drop is not observed in UGC03957 and the gas mass fraction profile stays below the cosmic mean value up to ~<em>R</em>₂₀₀ which points to a possible difference between clusters and groups. <br /> The second project of this work addresses galaxy clusters as cosmological probes and was submitted to <em>Astronomy & Astrophysics</em> in April 2017. Relaxed distant clusters are essential for tests of the cosmological standard model using the gas mass fraction. Therefore, the dynamical status is crucial and X-ray observations of the ICM are the prime tool to investigate the hydrodynamical properties. Here, the extremely X-ray luminous high redshift cluster Cl<em>J</em>120958.9+495352 (<em>z</em> = 0.902) is studied with the XMM-Newton satellite and the Hubble Space Telescope. The results show that this object is one of the most luminous clusters known with <em>L</em>_X = (18.7 ± 1.3) × 10⁴⁴ erg/s in the 0.1 - 2.4 keV band. Additionally, strong indications for the presence of a cool core are found from the temperature profile and the central cooling time which makes this rare cluster a valuable object for cosmological probes. A gas mass fraction of <em>f</em>_gas,2500 = 0.11 ± 0.05 is obtained in good agreement with previous findings and the standard ΛCDM cosmology. <br /> In the last project, the imprints of structure formation in the ICM are studied in detail for the disturbed cluster A2163 with the Suzaku satellite. A2163 likely underwent one or several merging processes in the recent past and the work at hand reveals several shock fronts in the ICM. So far, only relatively few shocks have been detected and studied in detail in X-rays. In this work, the gas properties in two azimuthal directions are investigated yielding significant differences with higher surface brightness and emission measure profiles in north-east (NE) and a lower temperature in south-west (SW) direction. In both directions, a shock front at <em>R</em>~1.3 Mpc is found, visible as distinct jumps in the temperature profiles with Mach numbers of <em>M</em> = 1.5 ± 0.2 and <em>M</em> = 3.3 ± 0.8 and shock velocities of <em>v</em> = (1.2 ± 0.2) × 10³ km/s and <em>v</em> = (4.1 ± 1.0) × 10³ km/s, for the NE and SW direction, respectively. The former is a typical value for X-ray detected merging shocks while the latter is comparable to one of the strongest known shocks, measured in the Bullet cluster. The SW density and temperature profile exhibit evidence for a second shock front at <em>R</em>~700 kpc with <em>M</em> = 1.8 ± 1.2, coinciding with a steepening of the surface brightness profile and spatially close to a cool core “bullet”. Additionally, spatial correlation to radio emission is found, in particular a coinciding of the NE shock front with a radio relic which hints at a causal connection of these phenomena, likely due to relativistic electrons which are accelerated in the shock.