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

TL;DR

This study uses 3D simulations to show that modest turbulence in galaxy cluster cores can prevent cooling catastrophes by suppressing magnetic instabilities, leading to a bimodal distribution of cluster states.

Contribution

It demonstrates that small variations in turbulence levels can trigger transitions between cool-core and non cool-core galaxy clusters, highlighting turbulence's key role.

Findings

Turbulence suppresses the heat-flux-driven buoyancy instability (HBI).

Small turbulence fluctuations can cause cluster state transitions.

Thermal conduction and turbulence together maintain thermal stability.

Abstract

We study the effects of externally imposed turbulence on the thermal properties of galaxy cluster cores, using three-dimensional numerical simulations including magnetic fields, anisotropic thermal conduction, and radiative cooling. The imposed "stirring" crudely approximates the effects of galactic wakes, waves generated by galaxies moving through the intracluster medium (ICM), and/or turbulence produced by a central active galactic nucleus. The simulated clusters exhibit a strong bimodality. Modest levels of turbulence, ~100 km/s (~10% of the sound speed), 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 (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 can initiate transitions between cool-core (low entropy) and non cool-core (high entropy) states.