Baryon Census in Hydrodynamical Simulations of Galaxy Clusters
  We carry out an analysis of a set of cosmological SPH hydrodynamical
simulations of galaxy clusters and groups aimed at studying the total baryon
budget in clusters, and how this budget is shared between the hot diffuse
component and the stellar component. Using the TreePM+SPH GADGET-3 code, we
carried out one set of non-radiative simulations, and two sets of simulations
including radiative cooling, star formation and feedback from supernovae (SN),
one of which also accounting for the effect of feedback from active galactic
nuclei (AGN). The analysis is carried out with the twofold aim of studying the
implication of stellar and hot gas content on the relative role played by SN
and AGN feedback, and to calibrate the cluster baryon fraction and its
evolution as a cosmological tool. We find that both radiative simulation sets
predict a trend of stellar mass fraction with cluster mass that tends to be
weaker than the observed one. However this tension depends on the particular
set of observational data considered. Including the effect of AGN feedback
alleviates this tension on the stellar mass and predicts values of the hot gas
mass fraction and total baryon fraction to be in closer agreement with
observational results. We further compute the ratio between the cluster baryon
content and the cosmic baryon fraction, Y_b, as a function of cluster-centric
radius and redshift. At R_500 we find for massive clusters with
M_500>2\times10^{14} h^{-1} M_sun that Y_b is nearly independent of the
physical processes included and characterized by a negligible redshift
evolution: Y_{b,500}=0.85+/-0.03 with the error accounting for the intrinsic
r.m.s. scatter within the set of simulated clusters. At smaller radii, R_2500,
the typical value of Y_b slightly decreases, by an amount that depends on the
physics included in the simulations, while its scatter increases by about a
factor of two.
