Gravitational diffusion in the intra-cluster medium

Abstract

We revisit the process of gravitational sedimentation of helium and heavy elements in the intra-cluster medium. We find that helium applies an inward drag force on heavy elements, boosting their sedimentation speed to nearly half its own. This speed is almost independent of the mass and the electric charge of heavy elements. In the absence of small-scale magnetic fields, helium sedimentation can increase the He/H abundance ratio in the cores of hot clusters by three orders of magnitude. It also steepens the baryonic density profile yielding a higher X-ray luminosity, which offers an explanation of the observed luminosity-temperature relation. If the primordial He/H ratio is assumed, then the gas density inferred from the observed X-ray emissivity might be underestimated by 30% in the cores of clusters and overestimated by 7% in the outer regions. The dark matter density on the other hand might be overestimated by a factor of 8/3 in the cores and underestimated by 18% in outer regions.

4 pages, 3 figures

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• The UV-upturn in brightest cluster galaxies
• Effect of Helium Sedimentation on X-ray Measurements of Galaxy Clusters
• Helium Sedimentation and the UV Upturn in Brightest Cluster Galaxies
• A Hot Helium Plasma in the Galactic Center Region
• Thermal diffusion in the IGM of clusters of galaxies
• The Effect of Helium Sedimentation on Galaxy Cluster Masses and Scaling Relations
• The Stability of Weakly Collisional Plasmas with Thermal and Composition Gradients
• Plasma Instabilities in the Context of Current Helium Sedimentation Models: Dynamical Implications for the ICM in Galaxy Clusters
• On Helium Mixing in Quasi-global Simulations of the Intracluster Medium
• Impact of thermal diffusion and other abundance anomalies on cosmological uses of galaxy clusters
• Local Simulations of Instabilities Driven by Composition Gradients in the ICM
• On the Helium fingers in the intracluster medium
• Effect of Composition Gradient on Magnetothermal Instability Modified by Shear and Rotation