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 having a comoving size of 410 h-1 Mpc, which have been carried out with the TREE-PM/smoothed particle hydrodynamics code GADGET-3, for a Wilkinson Microwave Anisotropy Probe-7 Λ cold dark matter cosmological model. Besides a dark matter (DM)-only simulation, we also carry out two hydrodynamical simulations: the first one includes non-radiative physics, with gas heated only by gravitational processes; the second one includes radiative cooling, star formation and kinetic feedback in the form of galactic ejecta triggered by supernova explosions. All simulations follow the evolution of two populations of 10243 particles each, with mass ratio such that to reproduce the assumed baryon density parameter, with the population of lighter particles assumed to be collisional in the hydrodynamical runs. We identified haloes using a spherical overdensity algorithm and their masses are computed at three different overdensities (with respect to the critical one), Δc= 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 haloes more massive than ?. In this range, mass increase in the hydrodynamical simulations is of about 4-5 per cent at Δc= 500 and ˜1-2 per cent at Δc= 200. Quite interestingly, these differences are nearly the same for both radiative and non-radiative simulations. Mass variations depend on halo mass and physics included for higher overdensity, Δc= 1500, and smaller masses. Such variations of halo masses induce corresponding variations of the halo mass function (HMF). At z= 0, the HMFs for gravitational heating and cooling and star formation simulations are close to the DM one, with differences of ≲3 per cent at Δc= 200, and ≃7 per cent at Δc= 500, with ˜10-20 per cent differences reached at Δc= 1500. At this higher overdensity, the increase of the HMF for the radiative case is larger by about a factor of 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, with a scatter of ≲3 per cent at Δc= 500 and ≲2 per cent at Δc= 200. Our results have interesting implications for assessing uncertainties in the mass function calibration associated with the uncertain baryon physics, in view of cosmological applications of future large surveys of galaxy clusters.