The origin of ICM enrichment in the outskirts of present-day galaxy clusters from cosmological hydrodynamical simulations

Abstract: The uniformity of the intra-cluster medium (ICM) enrichment level in the outskirts of nearby galaxy clusters suggests that chemical elements were deposited and widely spread into the intergalactic medium before the cluster formation. This observational evidence is supported by numerical findings from cosmological hydrodynamical simulations, as presented in Biffi et al. (2017), including the effect of thermal feedback from active galactic nuclei. Here, we further investigate this picture, by tracing back in time the spatial origin and metallicity evolution of the gas residing at z=0 in the outskirts of simulated galaxy clusters. In these regions, we find a large distribution of iron abundances, including a component of highly-enriched gas, already present at z=2. At z>1, the gas in the present-day outskirts was distributed over tens of virial radii from the the main cluster and had been already enriched within high-redshift haloes. At z=2, about 40% of the most Fe-rich gas at z=0 was not residing in any halo more massive than 1e11 Msun/h in the region and yet its average iron abundance was already 0.4, w.r.t. the solar value by Anders & Grevesse (1989). This confirms that the in situ enrichment of the ICM in the outskirts of present-day clusters does not play a significant role, and its uniform metal abundance is rather the consequence of the accretion of both low-metallicity and pre-enriched (at z>2) gas, from the diffuse component and through merging substructures. These findings do not depend on the mass of the cluster nor on its core properties.