Plasma Instabilities in the Context of Current Helium Sedimentation Models: Dynamical Implications for the ICM in Galaxy Clusters

Abstract

Understanding whether Helium can sediment to the core of galaxy clusters is important for a number of problems in cosmology and astrophysics. All current models addressing this question are one-dimensional and do not account for the fact that magnetic fields can effectively channel ions and electrons, leading to anisotropic transport of momentum, heat, and particle diffusion in the weakly collisional intracluster medium (ICM). This anisotropy can lead to a wide variety of instabilities, which could be relevant for understanding the dynamics of heterogeneous media. In this paper, we consider the radial temperature and composition profiles as obtained from a state-of-the-art Helium sedimentation model and analyze its stability properties. We find that the associated radial profiles are unstable, to different kinds of instabilities depending on the magnetic field orientation, at all radii. The fastest growing modes are usually related to generalizations of the Magnetothermal Instability (MTI) and the Heat-flux-driven Buoyancy Instability (HBI) which operate in heterogeneous media. We find that the effect of sedimentation is to increase (decrease) the predicted growth rates in the inner (outer) cluster region. The unstable modes grow fast compared to the sedimentation timescale. This suggests that the composition gradients as inferred from sedimentation models, which do not fully account for the anisotropic character of the weakly collisional environment, might not be very robust. Our results emphasize the subtleties involved in understanding the gas dynamics of the ICM and argue for the need of a comprehensive approach to address the issue of Helium sedimentation beyond current models.

Accepted by ApJ. 15 pages, 17 figures

References in corpus (9)

• Cosmology and Astrophysics from Relaxed Galaxy Clusters II: Cosmological Constraints
• Hydrodynamic Simulation of Non-thermal Pressure Profiles of Galaxy Clusters
• Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas
• Anisotropic Thermal Conduction and the Cooling Flow Problem in Galaxy Clusters
• Nonlinear Simulations of the Heat Flux Driven Buoyancy Instability and its Implications for Galaxy Clusters
• Saturation of the Magnetothermal Instability in Three Dimensions
• The Magnetothermal Instability in the Intracluster Medium
• Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction
• The Stability of Weakly Collisional Plasmas with Thermal and Composition Gradients

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• On Helium Mixing in Quasi-global Simulations of the Intracluster Medium
• Local Simulations of Instabilities Driven by Composition Gradients in the ICM
• Electron Re-acceleration via Ion Cyclotron Waves in the Intracluster Medium
• Stability Analysis of Convection in the Intracluster Medium
• On the Helium fingers in the intracluster medium
• Effect of Composition Gradient on Magnetothermal Instability Modified by Shear and Rotation