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Biffi, Veronica (2012): Studying the ICM velocity structure within galaxy clusters with simulations and X-ray observations. Dissertation, LMU München: Faculty of Physics
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Abstract

Galaxy clusters are optimal laboratories to test cosmology as well as models for physical processes acting on smaller scales. X–ray observations of the hot gas filling their dark matter potential well, i.e. the intra–cluster medium (ICM), still provides one of the best ways to investigate the intrinsic properties of clusters. Methods based on X–ray observations of the ICM are commonly used to estimate the total mass, assuming that the gas traces the underlying potential well and satisfies spherical symmetry, and thermal motions dominate the total pressure support. However, non–thermal motions are likely to establish in the ICM, hence, contribute to the total pressure and have to be taken into account in the mass estimate. In this thesis I study the ICM thermo–dynamical structure by combining hydrodynamical simulations and synthetic X–ray observations of galaxy clusters. The main goal is to study their gas velocity field and the implications due to non–thermal motions: first, by analysing directly the velocity patterns in simulated clusters and, secondly, by reconstructing the internal ICM structure from mock X–ray spectra. To this aim, I developed and applied an X–ray photon simulator to obtain synthetic X–ray spectra from the gas component in hydrodynamical simulations of galaxy clusters. The main findings of this work are as follows. (i) Ordered, rotational patterns in the gas velocity field in cluster cores can establish during the mass assembly process, but are found to be transient phenomena, easily destroyed by passages of gas–rich subhaloes. This suggests that in smoothly growing haloes the phenomenon is in general of minor effect. Nonetheless, major mergers or highly disturbed systems can indeed develop significant ordered motions and rotation, which contribute up to 20% to the total mass. (ii) It is indeed possible to reconstruct the thermal structure of the ICM in clusters from X–ray spectral analysis, by recovering the emission measure (EM) distribution of the gas as a function of temperature. This is possible with current X–ray telescopes (e.g. Suzaku) via multi–temperature fitting of X– ray spectra. (iii) High–precision X–ray spectrometers, such as ATHENA, will allow us to measure velocity amplitudes of ICM non–thermal motions, from the velocity broadening of heavy–ion (e.g. iron) emission lines. In this work, these achievements are obtained by applying the virtual X–ray simulator to generate ATHENA synthetic spectra of simulated clusters. The non–thermal velocity of the ICM in the central region is used to further characterise the cluster and the level of deviation from the expected self–similarity. By excluding the clusters with the highest non–thermal velocity dispersion, the scatter of the LX −T relation for the sample is significantly reduced, which will allow for a more precise comparison between observations and simulations.