Gas cooling in semi-analytic models and SPH simulations: are results
  consistent?
  We present a detailed comparison between the galaxy populations within a
massive cluster, as predicted by hydrodynamical SPH simulations and by a
semi-analytic model (SAM) of galaxy formation. Both models include gas cooling
and a simple prescription of star formation, which consists in transforming
instantaneously any cold gas available into stars, while neglecting any source
of energy feedback. We find that, in general, galaxy populations from SAMs and
SPH have similar statistical properties, in agreement with previous studies.
However, when comparing galaxies on an object-by-object basis, we find a number
of interesting differences: a) the star formation histories of the brightest
cluster galaxies (BCGs) from SAM and SPH models differ significantly, with the
SPH BCG exhibiting a lower level of star formation activity at low redshift,
and a more intense and shorter initial burst of star formation with respect to
its SAM counterpart; b) while all stars associated with the BCG were formed in
its progenitors in the semi-analytic model used here, this holds true only for
half of the final BCG stellar mass in the SPH simulation, the remaining half
being contributed by tidal stripping of stars from the diffuse stellar
component associated with galaxies accreted on the cluster halo; c) SPH
satellites can loose up to 90 per cent of their stellar mass at the time of
accretion, due to tidal stripping, a process not included in the semi-analytic
model used in this study; d) in the SPH simulation, significant cooling occurs
on the most massive satellite galaxies and this lasts for up to 1 Gyr after
accretion. This physical process is not included in the semi-analytic model
used in our study, as well as in most of the models discussed in the recent
literature.
