Statistical analysis of the X-ray morphology of galaxy clusters
The morphological analysis of galaxy clusters in X-rays allows a reliable determination of their dynamical state. Substructures on (sub-)Mpc scale influence the gravitational potential of a cluster and manifest themselves in the X-ray surface brightness distribution as secondary peaks or overall irregular shape. They lead to deviations from the hydrostatic equilibrium and spherical shape, two assumptions which are widely used in galaxy cluster studies to derive global astrophysical properties.
... ysical properties. Analyzing the X-ray morphology of clusters thus yields valuable information, provided that the employed substructure measures are well-tested and well-calibrated. In this work, the X-ray morphology of galaxy clusters is quantified using three common substructure parameters (power ratios, center shift and the asymmetry parameter), which are subsequently employed to study the disturbed cluster fraction as a function of redshift. To ensure a reliable application of these substructure parameters on a variety of X-ray images, a detailed parameter study is conducted. It focuses on the performance and reliability of the parameters for varying data quality using simulated and observed X-ray images. In particular, when applying them to X-ray images with low photon counts such as observations of distant clusters or survey data, it is important to know the characteristics of the parameters. Comparing the three substructure measures, the center shift parameter is most robust against Poisson noise and allows a reliable determination of the clusters' dynamical state even for low-count observations. Power ratios, especially the hexapole P3/P0, and the asymmetry parameter, on the other hand, are severely affected by noise, which results in spuriously high substructure signals. Furthermore, this work presents methods to minimize the noise bias. The results of the parameter study provide a step forward in the morphological analysis of high-redshift clusters and are employed in the framework of this thesis to quantify the evolution of the dis [...]