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