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 halos, but most calibrations to date have ignored complex gas physics associated with halo baryons. We explore the sensitivity of the halo mass function to baryon physics using two pairs of gas-dynamic simulations that are likely to bracket the true behavior. 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 re-simulations of the original 500 Mpc/h Millennium Simulation of Springel et al. (2005), run with the SPH code Gadget-2 and using a refined physics treatment approximated by preheating (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 ~10^{14} Msun/h halos in the PH and CSF treatments are shifted by averages of -15 \pm 1 percent and +12 \pm 5 percent, respectively. These mass shifts cause ~ 30% deviations in number density relative to the Tinker function, significantly larger than the 5% statistical uncertainty of that calibration. 
R. Stanek, D. Rudd, and A. E. Evrard