Gravitational diffusion in the intracluster medium

Abstract

We revisit the process of gravitational sedimentation of helium and heavy elements in the intracluster 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 per cent in the cores of clusters and overestimated by 7 per cent in the outer regions. The dark matter density, on the other hand, might be overestimated by a factor of 2.3 in the cores and underestimated by 18 per cent in the outer regions.