Suppressed heat conductivity in the intracluster medium: implications for the magneto-thermal instability

TL;DR

This study investigates how microscale instabilities that suppress heat conductivity affect the magneto-thermal instability in the intracluster medium, finding that the nonlinear saturation of the MTI remains largely unaffected.

Contribution

The paper introduces a subgrid model to simulate the impact of microscale instabilities on heat conductivity and assesses their effect on the nonlinear evolution of the MTI.

Findings

Nonlinear saturation of the MTI is robust despite reduced heat conductivity.

Suppression of heat transport causes only modest changes in MTI saturation.

Microscale instabilities have limited impact on large-scale ICM dynamics.

Abstract

In the outskirts of the intracluster medium (ICM) in galaxy clusters, the temperature decreases with radius. Due to the weakly collisional nature of the plasma, these regions are susceptible to the magneto-thermal instability (MTI), which can sustain turbulence and provide turbulent pressure support in the ICM. This instability arises due to heat conduction directed along the magnetic field, with a heat conductivity which is normally assumed to be given by the Spitzer value. Recent numerical studies of the ion mirror and the electron whistler instability using particle-in-cell codes have shown that microscale instabilities can lead to a reduced value for the heat conductivity in the ICM. This could in turn influence the efficiency with which the MTI drives turbulence. In this paper we investigate the influence of reduced heat transport on the nonlinear evolution of the MTI. We study plane-parallel, initially static atmospheres and employ a subgrid model that mimics the influence of the mirror instability on the heat conductivity. We use this subgrid model to assess the effect of microscales on the large scale dynamics of the ICM. We find that the nonlinear saturation of the MTI is surprisingly robust in our simulations. Over a factor of $\sim 10^3$ in the thermal-to-magnetic pressure ratio and collisionality we find at most modest changes to the saturation of the MTI with respect to reference simulations where heat transport is unsuppressed.