Impact of thermal diffusion and other abundance anomalies on cosmological uses of galaxy clusters

Abstract

Depending on the topology of the magnetic field and characteristics of turbulent motions, diffusion can significantly affect the distribution of elements, in particular helium, in the intracluster medium (ICM). As has been noted previously, an incorrect assumption about the helium abundance will lead to an error in the iron abundance determined from X-ray spectroscopy. The corresponding effect on the temperature measurement is negligibly small. An incorrectly assumed helium abundance will also lead to a systematic error in angular distance measurements based on X-ray and Sunyaev-Zeldovich (SZ) observations of clusters of galaxies. Its magnitude is further amplified by the associated error in the metal abundance determination, the impact being larger at lower ICM temperatures. Overall, a factor of 2-5 error in the helium abundance will lead to an ≈10-25 per cent error in the angular distance. We solve the full set of Burgers equations for a multicomponent intracluster plasma to determine the maximal effect of diffusion on the interpretation of X-ray and microwave observations of clusters of galaxies. For an isothermal cluster, gravitational sedimentation can lead to up to a factor of ∼5-10 enhancements of helium and metal abundances in the cluster centre on a ∼3-7 Gyr time-scale. In cool-core clusters in contrast, thermal diffusion can counteract gravitational sedimentation and effectively remove helium and metals from the cluster's inner core. In either case, a significant, up to ≈40 per cent, error in the metal abundances determined by means of X-ray spectroscopy is possible. The angular distance determined from X-ray and SZ data can be underestimated by up to ≈10-25 per cent.