Abstract

The assumption of a spatially uniform helium distribution in the intracluster medium (ICM) 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 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. For this Astrophysics Seminar I will present a quasi-global approach that we use to simulate the mixing of helium as induced by a generalization of the heat-flux-driven buoyancy instability (HBI), which feeds off thermal and composition gradients. In the inner region of the ICM , mixing can occur on a time scale of a few Gyrs, which suggests that composition gradients in cluster cores might be shallower than one-dimensional models predict.