From Plasma Microphysics to Global Dynamics in Clusters of Galaxies: Turbulence, Magnetic Fields, and Plasma Instabilities

Event Date: 

Wednesday, May 23, 2012 - 4:00pm

Event Location: 

  • Broida 3302

Event Contact: 

Speaker:

Matthew Kunz, Princeton

The intracluster medium (ICM) of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Assessing the efficacy of this transport is a vital step in understanding how the ICM avoids catastrophic cooling, why cool-core clusters exhibit remarkably similar temperature profiles, and what generates and sustains the observationally inferred turbulent velocity and magnetic fields. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. The local changes in magnetic field strength that attend these instabilities cause pressure anisotropies that viscously damp motions parallel to the magnetic field. In this talk, I will discuss two important effects of pressure anisotropy on the dynamical and thermal stability of the ICM. First, by stifling the convergence/divergence of magnetic field lines, pressure anisotropy significantly affects how the ICM interacts with the temperature gradient. Instabilities which depend upon convergence/divergence of magnetic field lines to generate unstable buoyant motions (the HBI) are suppressed, whereas those which are otherwise impeded by field-line convergence/divergence (the MTI) are strengthened. Second, because the viscous heating of the ICM is regulated by the pressure anisotropy -- which itself is nonlinearly regulated by the plasma beta parameter via rapidly-growing microscale instabilities -- pressure anisotropy may play a crucial role in mitigating cooling flows and preventing cluster core collapse. I will discuss the physical interpretation of these effects in detail, placing them within the larger context of formulating a pragmatic analytical and numerical framework for modeling astrophysical multi-scale plasma dynamics. Observationally testable predictions will be made.