טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSternberg Assaf
SubjectInflating Radio Bubbles in the Cores of Galaxy Clusters
DepartmentDepartment of Physics
Supervisor Professor Noam Soker
Full Thesis textFull thesis text - English Version


Abstract

            Galaxy clusters are the largest gravitationally bound and relaxed systems in the Universe. They consist of galaxies, inter-galactic gas (the intra-cluster-medium or ICM), and dark matter. The ICM is a hot gas at a few 107 K. This gas emits radiation, Bremsstrahlung and X-ray lines. As the gas radiates it should cool to low temperatures of the order of 104 K and flow towards the cluster's center (termed the cooling flow). There is a large discrepancy between the rate of cooling observed in galaxy clusters and the rate that is calculated in theory (termed the cooling flow problem). One of the widely accepted solutions for this problem is that there is some kind of heating mechanism that offsets most of the cooling. Many galaxy clusters posses X-ray deficient cavities, which contain radio emission, in their inner part, that are termed radio bubbles. Based on observations, it is inferred that these cavities are filled with hot-low density matter. It is widely accepted that these bubbles are inflated by jets blown by the active galactic nucleus (AGN) of the cD galaxy. Some observations also show higher X-ray emissivity arcs, called ripples, which are thought to be sound waves or weak shocks. Age estimates of many bubbles show them to be older than the time it would have taken the Rayleigh-Taylor instability to destroy them. It is widely accepted that these bubbles play a crucial role in a plausible solution of the cooling flow problem. This has led many authors to try and study their inflation and evolution via simulations. These authors used artificial bubbles, i.e., cavities that were inserted onto the computational grid by a predetermined numerical recipe, that could not reproduce some physical properties of the bubbles and their effect on the ICM. Therefore, many authors imposed stabilizing magnetic fields, viscosity, repeated bubble inflation episodes, etc, to get results in better agreement with the observations. This thesis presents the results of my Ph.D. research project in which it was shown that radio bubbles can be inflated near the cluster's center by wide or narrow precessing jets. It was also shown that using a more realistic bubble inflation scenario causes features, which are absent when artificial bubbles are used and are crucial in explaining and reproducing many of the observed properties, to appear without the need to impose artificial features and behaviors that are not mandatory.