Local Simulations of Instabilities Driven by Composition Gradients in the ICM

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.

13 pages, 13 figures, published in ApJ, animation of Figure 7 at http://tberlok.dk/movies/icm_local.html

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Cited by in corpus (8)

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• Radial metal abundance profiles in the intra-cluster medium of cool-core galaxy clusters, groups, and ellipticals
• Suppressed heat conductivity in the intracluster medium: implications for the magneto-thermal instability
• Braginskii viscosity on an unstructured, moving mesh accelerated with super-time-stepping
• On Helium Mixing in Quasi-global Simulations of the Intracluster Medium
• Dynamical properties and detectability of the magneto-thermal instability in the intracluster medium
• On the Helium fingers in the intracluster medium
• Effect of Composition Gradient on Magnetothermal Instability Modified by Shear and Rotation