The effects of baryons on the halo mass function
  We present an analysis of the effects of baryon physics on the halo mass
function. The analysis is based on simulations of a cosmological volume.
Besides a Dark Matter (DM) only simulation, we also carry out two other
hydrodynamical simulations. We identified halos using a spherical overdensity
algorithm and their masses are computed at three different overdensities (with
respect to the critical one), $\Delta_c=200$, 500 and 1500. We find the
fractional difference between halo masses in the hydrodynamical and in the DM
simulations to be almost constant, at least for halos more massive than $\log
(M_{\Delta_c} / \hMsun)\geq 13.5$. In this range, mass increase in the
hydrodynamical simulations is of about 4-5 per cent at $\Delta_c=500$ and $\sim
1$ - 2 per cent at $\Delta_c=200$. Quite interestingly, these differences are
nearly the same for both radiative and non-radiative simulations. Such
variations of halo masses induce corresponding variations of the halo mass
function (HMF). At $z=0$, the HMFs for GH and CSF simulations are close to the
DM one, with differences of $\mincir 3$ per cent at $\Delta_c = 200$, and
$\simeq 7$ per cent at $\Delta_c=500$, with $\sim 10$ - 20 per cent differences
reached at $\Delta_c = 1500$. At this higher overdensity, the increase of the
HMF for the radiative case is larger by about a factor 2 with respect to the
non--radiative case. Assuming a constant mass shift to rescale the HMF from the
hydrodynamic to the DM simulations, brings the HMF difference with respect to
the DM case to be consistent with zero. Our results have interesting
implications to bracket uncertainties in the mass function calibration
associated to the uncertain baryon physics, in view of cosmological
applications of future large surveys of galaxy clusters. (Abridged)
