Magnetic fields and the cluster cooling flow hypothesis

Through the development of modern observations and computational techniques, the picture associated with clusters of galaxies has become more complex. Initially thought to be isothermal and hydrostatic, the X-ray emitting intracluster medium (ICM) is now viewed as a turbulent, magnetized environment. Mergers, ejecta from member galaxies, and jets from active galactic nuclei all serve to perturb the ICM. Magnetic field has been detected at all spatial scales within the ICM, with magnetic pressure close to equipartition with the gas pressure near some cluster cores. All of these phenomena have an impact on the ICM. Nearly thirty years ago a subset of clusters of galaxies referred to as ``cooling flows'' were identified through enhanced X-ray emission above the typical cluster sample. The radiative cooling time for these systems was found to be shorter than the cluster age. It was hypothesized that radiative cooling should bring the ICM out of hydrostatic balance, evolving into a steady-state radial inflow. Since direct observation of material inflow is not yet possible, estimates of the mass accretion rate have come from X-ray observations. The dominant mechanism governing the dynamics of these environments was assumed to be purely hydrodynamic. The role which magnetic fields play within the ICM has come into question due to magnetic field measurements within the cores of ``cooling flows.'' Previous attempts have been made to address the importance of magnetic fields within the ICM, however, the conclusions have been conflicting. We investigate the impact which magnetic fields have on a radiatively collapsing environment using one-dimensional numerical simulations. A hydrodynamic baseline is first established, and then followed by simulations invoking ideal magnetohydrodynamics (MHD) as well as magnetic dissipation. The results from the simulations indicate that hydrodynamics alone will not produce a solution which matches the observed characteristics of cooling flows for all times. The hydrodynamic simulations indicate a period of catastrophic cooling, immediately followed by catastrophic collapse of the environment. Incorporating magnetic fields into the solutions changes the evolution as an intermediate phase of non-thermal pressure persists for several Gigayears. During this period of time the X- ray surface brightness profile matches X-ray observations of cluster ``cooling flows,'' however the estimated rotation measures are high with respect to observations. Dissipation of magnetic fields via reconnection has been suggested as a means of influencing the dynamics. We investigated this using three different rates. As magnetic field is dissipated, the plasma is heated. Given enough time, cooling will overcome heating. If the magnetic field is dissipated rapidly, then the resulting environment has a short-lived non-thermal pressure support phase, eventually evolving much like the hydrodynamic simulations.