The entropy core in galaxy clusters: numerical and physical effects in
  cosmological grid simulations
  We investigated the numerical and physical reasons leading to a flat
distribution of low gas entropy in the core region of galaxy clusters, as
commonly found in grid cosmological simulations. To this end, we run a set of
30 high resolution re-simulations of a 3 x 10^14 M_sol/h cluster of galaxies
with the AMR code ENZO, exploring and investigating the details involved in the
production of entropy in simulated galaxy clusters. The occurrence of the flat
entropy core is found to be mainly due to hydro-dynamical processes resolved in
the code and that additional spurious effects of numerical origin (e.g.
artificial heating due to softening effects or N-body noise) can affect the
size and level of the entropy core only in a minor way. We show that the
entropy profile of non-radiative simulations is produced by a mechanism of
"sorting in entropy" which takes place with regularity during the cluster
evolution. Using gas tracers we prove that the flat entropy core is caused by
physical mixing of gravity-driven subsonic motions within the shallow inner
cluster potential. Re-simulations were also produced for the same cluster
object with the addition of radiative cooling, uniform pre-heating at high
redshift (z=10) and late (z<1) thermal energy feedback from AGN activity in the
cluster, in order to assess the effects of such mechanisms on the final entropy
profile of the cluster. We report on the infeasibility of balancing the
catastrophic cooling and recovering a flat entropy profile with the
investigated trials for AGN activity alone, while for a sub-set of pre-heating
models, or AGN feedback plus pre-heating models, a flat entropy distribution
similar to non-radiative runs can be obtained with a viable energy requirement,
and in good consistency with X-ray observations.
