Galaxies in the central regions of simulated galaxy clusters
  In this paper, we assess the impact of numerical resolution and of the
implementation of energy input from AGN feedback models on the inner structure
of cluster sub-haloes in hydrodynamic simulations. We compare several zoom-in
re-simulations of a sub-sample of the cluster-sized haloes studied in
Meneghetti et al. (2020), obtained by varying mass resolution, softening length
and AGN energy feedback scheme. We study the impact of these different setups
on the subhalo abundances, their radial distribution, their density and mass
profiles and the relation between the maximum circular velocity, which is a
proxy for subhalo compactness. Regardless of the adopted numerical resolution
and feedback model, subhaloes with masses Msub < 1e11Msun/h, the most relevant
mass-range for galaxy-galaxy strong lensing, have maximum circular velocities
~30% smaller than those measured from strong lensing observations of Bergamini
et al. (2019). We also find that simulations with less effective AGN energy
feedback produce massive subhaloes (Msub> 1e11 Msun/h ) with higher maximum
circular velocity and that their Vmax - Msub relation approaches the observed
one. However the stellar-mass number count of these objects exceeds the one
found in observations and we find that the compactness of these simulated
subhaloes is the result of an extremely over-efficient star formation in their
cores, also leading to larger-than-observed subhalo stellar mass. We conclude
that simulations are unable to simultaneously reproduce the observed stellar
masses and compactness (or maximum circular velocities) of cluster galaxies.
Thus, the discrepancy between theory and observations that emerged from the
analysis of Meneghetti et al. (2020) persists. It remains an open question as
to whether such a discrepancy reflects limitations of the current
implementation of galaxy formation models or the LCDM paradigm.
