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Rebusco, Paola (2007): Impact of supermassive black holes on galaxy clusters. Dissertation, LMU München: Faculty of Physics
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Abstract

In the semi-analytical work presented here the feedback from supermassive black holes on galaxy clusters is investigated. In particular we aim at providing simple diagnostics tools to constrain the characteristic velocities and spatial scales of the hot Intra Cluster Medium (ICM) motions. In the so-called "cold core'' clusters these motions are believed to be driven by the activity of a central black hole. The methods developed here, together with present-day and future observations, are designed to help to solve the puzzle of cooling flow clusters (see section $1.3$) and understand better the AGN/gas interaction in smaller systems (down to individual galaxies).\\ Clusters of galaxies are the largest gravitationally bound systems in the Universe: they are composed of hundreds to thousands of galaxies, moving in a deep potential well set by the dominating dark matter. The whole volume of clusters is filled with hot (temperature $\sim 10^7-10^8$~K) and rarefied (electron density $10^{-4}-10^{-1} {\rm cm^{-3}}$) gas. In such a high-temperature regime even heavy elements (e.g. silicon, sulfur, iron etc.) are highly ionized up to [H]- or [He]-like ions and they emit in bright lines with energies from $\sim 0.7$ to $\sim 8$ keV. Using X-ray observations one can reliable measure all the major gas properties: the temperature, density and abundance of heavy elements.\\ A significant fraction of clusters (called "cool core'' clusters) show distinct signatures in the central region: the gas temperature drops inward, while the gas density increases. The central gas radiative cooling time in such clusters is much shorter than the age of the cluster and without any external source of energy the gas would cool well below X-ray temperatures. However observations suggest that the gas temperature drops only to 1-2 keV. One plausible explanation of this problem is that the activity of a central supermassive black hole deposits large amounts of mechanical energy into the cluster gas and that this balances the gas radiative losses. A direct implication of this hypothesis is that the hot gas is not at rest, but it is continuously stirred by the AGN activity.\\ The same class of cool core clusters is characterized by a centrally peaked distribution of the heavy elements abundance (usually measured using the He-like iron 6.7 keV line). The peaked abundance profiles are likely associated with the metals ejection by the stars of very massive elliptical galaxies, that are always present at the centers of these clusters. However the observed abundance distributions are significantly broader than the central galaxy light profiles, suggesting that some gas motions are spreading the metals ejected from the galaxy. We treat this process in a diffusion approximation to derive, from the X-ray observations, constraints on the characteristic velocities and spatial scales of the gas motions for a sample of cool core clusters and groups (Chapters $2$ and $3$). The parameters derived from a simple semi-analytic model are then compared with the results of numerical simulations of the AGN/gas interaction in the cluster core (Chapter $4$).\\ In Chapter $5$ we discuss the impact of the gas motions on the width of the strongest X-ray emission lines. Since the characteristic thermal velocities of heavy ions (e.g. iron) are much smaller than the sound speed of the gas, the width of the lines sensitively depends on the presence of gas motions. We show that both the absolute value of the linewidth and its dependence on the projected distance from the cluster center provide valuable diagnostics of the gas motions. Such measurements will soon become possible with the launch of X-ray micro-calorimeters in space.\\ This work has been done in collaboration with E.Churazov, R.Sunyaev, H.B\"ohringer, M.Br\"uggen, W.Forman and E.Roediger.