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We study the effects of externally imposed turbulence on the thermalproperties of galaxy cluster cores, using three-dimensional numericalsimulations including magnetic fields, anisotropic thermal conduction, andradiative cooling. The imposed "stirring" crudely approximates the effects ofgalactic wakes, waves generated by galaxies moving through the intraclustermedium (ICM), and/or turbulence produced by a central active galactic nucleus.The simulated clusters exhibit a strong bimodality. Modest levels ofturbulence, ~100 km/s (~10% of the sound speed), suppress the heat-flux-drivenbuoyancy instability (HBI), resulting in an isotropically tangled magneticfield and a quasi-stable, high entropy, thermal equilibrium with no coolingcatastrophe. Thermal conduction dominates the heating of the cluster core, butturbulent mixing is critical because it suppresses the HBI and (to a lesserextent) the thermal instability. Lower levels of turbulent mixing(approximately less than 100 km/s) 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 caninitiate transitions between cool-core (low entropy) and non cool-core (highentropy) states.

Turbulence in Galaxy Cluster Cores: a Key to Cluster Bimodality?