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