Magneto-Thermal Instability In Galaxy Clusters I: Theory and Two-Dimensional Simulations

TL;DR

This paper develops a theoretical framework and conducts two-dimensional simulations to understand the magneto-thermal instability (MTI) in galaxy clusters, revealing how it saturates and influences heat transport in the intracluster medium.

Contribution

It introduces a new theory for MTI saturation, explores the inverse cascade mechanism, and provides scalings for turbulence and heat flux in galaxy clusters.

Findings

Inverse cascade transfers kinetic energy to larger scales.

Entropy stratification limits eddy size and strength.

MTI effectively transports heat despite magnetic field complexity.

Abstract

Determining the origin of turbulence in galaxy clusters, and quantifying its transport of heat, is an outstanding problem, with implications for our understanding of their thermodynamic history and structure. As the dilute plasma of the intracluster medium (ICM) is magnetized, heat and momentum travel preferentially along magnetic field lines. This anisotropy triggers a class of buoyancy instabilities that destabilize the ICM, and whose turbulent motions can augment or impede heat transport. We focus on the magneto-thermal instability (MTI), which may be active in the periphery of galaxy clusters. We aim to take a fresh look at the problem and construct a general theory that explains the MTI saturation mechanism and provides scalings and estimates for the turbulent kinetic energy, magnetic energy, and heat flux. We simulate MTI turbulence with a Boussinesq code, SNOOPY, which, in contrast to previous work, allows us to perform an extensive sampling of the parameter space. In two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; at a characteristic "buoyancy scale", the energy is dumped into large-scale g-modes, which subsequently dissipate. Consequently, the entropy stratification sets an upper limit on the size and strength of turbulent eddies. Meanwhile, the MTI conveys a substantial fraction of heat, despite the tangled geometry of the magnetic field. In a companion paper, these results are extended to three-dimensional flows, and compared to real cluster observations.