On Helium Mixing in Quasi-global Simulations of the Intracluster Medium

TL;DR

This paper investigates helium mixing in the intracluster medium using quasi-global simulations, revealing that instabilities driven by thermal and composition gradients can significantly influence helium distribution and magnetic field orientation.

Contribution

It extends previous local theories by developing a quasi-global simulation approach to study helium mixing and magnetic field evolution in galaxy clusters.

Findings

Mixing occurs over a few Gyrs in cluster cores.

Magnetic field inclination averages around 45 degrees after mixing.

Composition gradients in cores may be shallower than 1D models predict.

Abstract

The assumption of a spatially uniform helium distribution in the intracluster medium can lead to biases in the estimates of key cluster parameters if composition gradients are present. The helium concentration profile in galaxy clusters is unfortunately not directly observable. Current models addressing the putative sedimentation are one-dimensional and parametrize the presence of magnetic fields in a crude way, ignoring the weakly-collisional, magnetized nature of the medium. When these effects are considered, a wide variety of instabilities can play an important role in the plasma dynamics. In a series of recent papers, we have developed the local, linear theory of these instabilities and addressed their non-linear development with a modified version of Athena. Here, we extend our study by developing a quasi-global approach that we use to simulate the mixing of helium as induced by generalizations of the heat-flux-driven buoyancy instability (HBI) and the magneto-thermal instability (MTI), which feed off thermal and composition gradients. In the inner region of the ICM, mixing can occur on few Gyrs, after which the average magnetic field inclination angle is $\sim 45^{\circ}$ resulting in an averaged Spitzer parameter higher by about 20 % than the value obtained in homogeneous simulations. In the cluster outskirts the instabilities are rather inefficient, due to the shallow gradients. This suggests that compositions gradients in cluster cores might be shallower than one-dimensional models predict. More quantitative statements demand more refined models that can incorporate the physics driving the sedimentation process and simultaneously account for the weakly-collisional nature of the plasma.