Turbulence and direct dark matter detection in the X-ray halo of galaxy clusters

This thesis consists of three main parts each of which deals with currently important problems in the research of clusters of galaxies. First I conducted a detailed study of 33 clusters with the aim of constraining turbulence in the hot phase of the intracluster medium (ICM). I demonstrate how the measured turbulence influences the mass measurements of clusters which is crucial for obtaining accurate cosmological constraints from cluster observations. In high-resolution X-ray observations of the hot plasma in clusters significant structures caused by AGN feedback, mergers, and turbulence can be detected. Many clusters have been observed with Chandra to great depths and at high resolution. Using archival data taken with the Chandra ACIS instrument I studied the thermodynamic perturbations of the X-ray emitting plasma and applied this to better understand the thermodynamic and dynamic state of the ICM. I analysed deep observations of a sample of 33 clusters with more than 100 ks of Chandra exposure each at distances between redshift 0.025 and 0.45. The combined exposure of the sample is 8 Ms. Fitting emission models to di↵erent regions of the extended X-ray emission I searched for perturbations in density, temperature, pressure, and entropy of the hot plasma. For individual clusters I mapped the thermodynamic properties of the ICM and measured their spread in circular concentric annuli. Comparing the spread of di↵erent gas quantities to high-resolution 3D hydrodynamic simulations, I constrain the average Mach number regime of the sample to Mach1D ⇡ 0.16 ± 0.07. In addition I found a tight correlation between metallicity, temperature and redshift with an average metallicity of Z ⇡ 0.3 ± 0.1 Z . This study provides detailed perturbation measurements for a large sample of clusters which can be used to study turbulence and make predictions for future X-ray observatories like eROSITA, Hitomi (Astro-H), and Athena. Secondly using the deep spectra from the clusters studied in the first part I obtained constraints on X-ray emission from a possible dark matter decay signal in the cluster halos. Recently an unidentified emission line at 3.55 keV has been detected in X-ray spectra of clusters of galaxies. The line has been discussed as a possible decay signature of 7.1 keV sterile neutrinos, which have been proposed as a dark matter candidate. For a sample of 33 high-mass clusters of galaxies I merged all observations from the Chandra data archive. The resulting high signal-to-noise spectra are used to constrain the flux of an unknown line emission at 3.55 keV in the individual spectra and a merged spectrum of all clusters. I do not find evidence for an unidentified emission line at 3.55 keV. The sample extends the list of objects searched for an emission line at 3.55 keV and will help to identify the xviii Zusammenfassung best targets for future studies of the potential dark matter decay line with upcoming X-ray observatories like Hitomi (Astro-H), eROSITA, and Athena. The third and last part of my work is to make predictions on how the upcoming eROSITA observatory will observe the clusters I studied. eROSITA will detect the largest sample of clusters to date (one hundred thousand expected) but at lower spatial resolution and shorter exposure time than the clusters I studied with Chandra. This makes it very important to simulate observations of well known clusters and identify possible bias in measured parameters. I simulated a large number of galaxy clusters with known temperature substructures and compared the results from analysing eROSITA simulated observations to results I obtained from Chandra. I was able to constrain the expected measurement bias in temperature and flux which is caused by substructures of the cluster emission. To get reliable cosmological parameter constraints from the eROSITA cluster sample it will be important to take into account the systematic bias found in this analysis. Chapter