Wednesday, August 4, 2010 - 3:15pm
Ian Parrish (Berkeley):
"The Interaction of Turbulence, Conduction, and Feedback in Galaxy Cluster Cores"
Abstract. We study the cooling flow problem in galaxy cluster cores in the context of including appropriate microphysics for thermal conduction, turbulence, and magnetic fields. In particular, we impose a "stirring" that crudely approximates the effects of galactic wakes, waves generated by galaxies moving through the intracluster medium (ICM), major and minor mergers, and/or turbulence produced by a central active galactic nucleus. The simulated clusters exhibit a strong bimodality. Modest levels of turbulence, ~100 km/s, suppress the heat-flux-driven buoyancy instability (HBI), resulting in an isotropically tangled magnetic field and a quasi-stable, high entropy, thermal equilibrium with no cooling catastrophe. Thermal conduction dominates the heating of the cluster core, but turbulent mixing is critical because it suppresses the HBI and (to a lesser extent) the thermal instability. Lower levels of turbulent mixing are insufficient to suppress the HBI, rapidly leading to a thermal runaway and a cool-core cluster. Remarkably, then, small fluctuations in the level of turbulence in galaxy cluster cores can initiate transitions between cool-core (low entropy) and non cool-core (high entropy) states. We investigate the possible connection between cluster merger history and the state of the cool cores and discuss the potential observable biases for X-ray cluster surveys.