Effects of Cooling and Star Formation on the Baryon Fractions in Clusters

Abstract

We study the effects of dissipation on the baryon fractions in clusters using high-resolution cosmological simulations of nine clusters that resolve formation of cluster galaxies. The simulations of each cluster are performed with the shock-capturing eulerian adaptive mesh refinement N-body+gasdynamics ART code with and without radiative cooling. We show that dissipation and associated galaxy formation increase the total baryon fractions within radii as large as the virial radius. The effect is the strongest within cluster cores, where the simulations with cooling have baryon fractions larger than the universal value, while the fraction of baryons in adiabatic simulations are smaller than universal. At larger radii (r >~ r_500) the cumulative baryon fractions in simulations with cooling are close to, while those in the adiabatic runs remain below than, the universal value. The gas fractions in simulations with dissipation are reduced by ~20-40% at r<0.3r_vir and ~10% at larger radii compared to the adiabatic runs, because a fraction of gas is converted into stars. There is an indication that gas fractions within different radii increase with increasing cluster mass as f_gas ~ M_vir^0.2. We find that the total baryon fraction within the virial radius does not evolve with time in both adiabatic simulations and in simulations with cooling. Finally, to evaluate systematic uncertainties in the baryon fraction in cosmological simulations we present a comparison of gas fractions in our adiabatic simulations to re-simulations of the same objects with the entropy-conserving SPH code Gadget. The cumulative gas fraction profiles in the two sets of simulations on average agree to 0.2, but differ systematically by up to 10% at small radii.