Local Simulations of Instabilities Driven by Composition Gradients in the ICM

TL;DR

This paper develops a modified MHD simulation to study how composition gradients in the ICM induce instabilities, leading to turbulence and magnetic field reorientation, which could impact cluster core properties and cooling flows.

Contribution

It introduces a new MHD code capable of simulating weakly-collisional plasmas with composition gradients, focusing on their instabilities and effects in the ICM.

Findings

Composition-driven instabilities induce turbulence.

Magnetic fields reorient to ~45° inclination.

Potential impact on cluster core insulation and cooling flows.

Abstract

The distribution of Helium in the intracluster medium (ICM) permeating galaxy clusters is not well constrained due to the very high plasma temperature. Therefore, the plasma is often assumed to be homogeneous. A non-uniform Helium distribution can however lead to biases when measuring key cluster parameters. This has motivated one-dimensional models that evolve the ICM composition assuming that the effects of magnetic fields can be parameterized or ignored. Such models for non-isothermal clusters show that Helium can sediment in the cluster core leading to a peak in concentration offset from the cluster center. The resulting profiles have recently been shown to be linearly unstable when the weakly-collisional character of the magnetized plasma is considered. In this paper, we present a modified version of the MHD code Athena, which makes it possible to evolve a weakly-collisional plasma subject to a gravitational field and stratified in both temperature and composition. We thoroughly test our implementation and confirm excellent agreement against several analytical results. In order to isolate the effects of composition, in this initial study we focus our attention on isothermal plasmas. We show that plasma instabilities, feeding off gradients in composition, can induce turbulent mixing and saturate by re-arranging magnetic field lines and alleviating the composition gradient. Composition profiles that increase with radius lead to instabilities that saturate by driving the average magnetic field inclination to roughly $45^{\circ}$. We speculate that this effect may alleviate the core insulation observed in homogeneous settings, with potential consequences for the associated cooling flow problem.