The effect of gas physics on the halo mass function

Cosmological tests based on cluster counts require accurate calibration of the space density of massive haloes, but most calibrations to date have ignored complex gas physics associated with halo baryons. We explore the sensitivity of the halomass function to baryon physics using two pairs of gas-dynamic simulations that are likely to bracket the true behaviour. Each pair consists of a baseline model involving only gravity and shock heating, and a refined physics model aimed at reproducing the observed scaling of the hot, intracluster gas phase. One pair consists of billion-particle resimulations of the original 500 h(-1) Mpc Millennium Simulation of Springel et al., run with the smoothed particle hydrodynamics (SPH) code GADGET-2 and using a refined physics treatment approximated by pre-heating (PH) at high redshift. The other pair are high-resolution simulations from the adaptive-mesh refinement code ART, for which the refined treatment includes cooling, star formation and supernova feedback (CSF). We find that, although the mass functions of the gravity-only (GO) treatments are consistent with the recent calibration of Tinker et al. (2008), both pairs of simulations with refined baryon physics show significant deviations. Relative to the GO case, the masses of similar to 10(14) h(-1) M(circle dot) haloes in the PH and CSF treatments are shifted by the averages of -15 +/- 1 and + 16 +/- 2 per cent, respectively. These mass shifts cause similar to 30 per cent deviations in number density relative to the Tinker function, significantly larger than the 5 per cent statistical uncertainty of that calibration.