Plasma turbulence in the intracluster medium of galaxy clusters: Magneto-immutability, micro-scale instabilities, and thermally stable heating

The hot, diffuse, and magnetized plasma that constitutes the intracluster medium of galaxy clusters (ICM) hosts turbulence that facilitates the transport of magnetic fields, metals, thermal energy, and cosmic rays in galaxy clusters. While this plasma is known to be only weakly collisional, its turbulence has primarily been studied using models that assume strict enforcement of local thermodynamic equilibrium (LTE) via rapid Coulomb collisions. As a result, critical non-LTE effects, like anisotropy in the thermal pressure and the resultant viscous stresses and kinetic micro-instabilities, are rarely accounted for. Using a combination of analytical and numerical methods, we present an investigation of ICM turbulence that explicitly incorporates the consequences of these non-LTE effects. In doing so, we build a theory of the spectrum of turbulent fluctuations that spans the weakly collisional and collisionless ranges of the cascade, as well as the transition between super- and sub-Alfvénic flows. We find that a self-organization process known as magneto-immutability enables the cascade to reach ion-Larmor scales – roughly 10 decades further than existing predictions – and simultaneously explains an apparent suppression of viscosity in observations of ICM turbulence. We also propose a model of viscous heating in ICM turbulence that is multi-phase in nature, in which one phase may establish a stable equilibrium with radiative Bremsstrahlung cooling. With recent studies of cosmic-ray transport in galaxy clusters, this points to an increasingly multi-phase picture of ICM dynamics.