Ph.D Thesis

Ph.D StudentShlomi Hillel
SubjectDynamics of Clumps in the Intracluster Medium
DepartmentDepartment of Physics
Supervisor Full Professors Soker Noam
Full Thesis textFull thesis text - English Version


The jet feedback mechanism is important in the understanding of many astrophysical phenomena, including galaxy formation and the evolution of cooling flow clusters. In the first part of the thesis we examine the accretion process. We argue that one of the basic assumptions of the Bondi accretion process, that the accreting object has zero pressure, might not hold in many galaxies because of the pressure exerted by stellar winds of stars orbiting the central supermassive black hole (SMBH). The winds of these high-velocity stars are shocked to temperatures above the virial temperature of the galaxy, leading to the formation of a hot bubble near the centre. This hot bubble can substantially reduce the mass accretion rate onto the SMBH. The accretion is likely mostly of cold clumps. In the second, main part of the thesis we simulate, first in 2D with axisymmetry and then in 3D, the evolution of the intracluster medium (ICM) of cooling flow clusters in response to multiple jet-activity cycles. We follow the thermal evolution of cold clumps and other regions in the ICM, and find that the main heating process of the clumps is mixing with a hot bubble of shocked jet gas, while shocks have a limited role. The inflation process of hot bubbles, which appear as X-ray deficient cavities in observations, is accompanied by complicated induced vortices inside and around the bubbles. The vorticity induces efficient mixing of the hot gas with the ICM and cool clumps, resulting in a substantial increase of the temperature and entropy of the clumps. For the parameters used by us the mixing process accounts for about four times as much heating as that by the kinetic energy in the ICM, namely, turbulence and sound waves. We conclude that turbulent heating plays a smaller role than mixing. Heating by shocks is even less efficient. Not all clumps are heated. Those that cool to very low temperatures will fall in and feed the central SMBH, hence closing the feedback cycle in what is termed the cold feedback mechanism.