BCG Mass Evolution in Cosmological Hydro-Simulations
  We analyze the stellar growth of Brightest Cluster Galaxies (BCGs) produced
by cosmological zoom-in hydrodynamical simulations of the formation of massive
galaxy clusters. The evolution of the stellar mass content is studied
considering different apertures, and tracking backwards either the main
progenitor of the $z=0$ BCG or that of the cluster hosting the BCG at $z=0$.
Both methods lead to similar results up to $z \simeq 1.5$. The simulated BCGs
masses at $z=0$ are in agreement with recent observations. In the redshift
interval from $z=1$ to $z=0$ we find growth factors 1.3, 1.6 and 3.6 for
stellar masses within 30kpc, 50kpc and 10% of $R_{500}$ respectively. The first
two factors, and in general the mass evolution in this redshift range, are in
agreement with most recent observations. The last larger factor is similar to
the growth factor obtained by a semi-analytical model (SAM). Half of the star
particles that end up in the inner 50 kpc was typically formed by redshift
$\sim$ 3.7, while the assembly of half of the BCGs stellar mass occurs on
average at lower redshifts $\sim 1.5$. This assembly redshift correlates with
the mass attained by the cluster at high $z \gtrsim 1.3$, due to the broader
range of the progenitor clusters at high-$z$. The assembly redshift of BCGs
decreases with increasing apertures. Our results are compatible with the {\it
inside-out} scenario. Simulated BCGs could lack intense enough star formation
(SF) at high redshift, while possibly exhibit an excess of residual SF at low
redshift.
