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

Abstract

We study the effects of radiative cooling and galaxy formation on the baryon fractions in clusters using high-resolution cosmological simulations that resolve formation of cluster galaxies. The simulations of nine individual clusters spanning a decade in mass are performed with the shock-capturing Eulerian adaptive mesh refinement N-body+gasdynamical Adaptive Refinement Tree code. For each cluster the simulations were done in the adiabatic regime (without dissipation) and with radiative cooling and several physical processes critical to various aspects of galaxy formation: star formation, metal enrichment, and stellar feedback. We show that radiative cooling of gas and associated star formation increase the total baryon fractions within radii as large as the virial radius. The effect is strongest within cluster cores, where the simulations with cooling have baryon fractions larger than the universal value, in contrast to the adiabatic simulations in which the fraction of baryons is substantially smaller than the universal value. At larger radii (r ≳ r500) the cumulative baryon fractions in simulations with cooling are close to the universal value. The gas fractions in simulations with dissipation are reduced by ≈20%-40% at r < 0.3rvir 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 fgas ∝ M. We find that the total baryon fraction within the cluster virial radius does not evolve with time in both adiabatic simulations and in simulations with cooling. The gas fractions in the latter decrease slightly from z = 1 to 0 due to ongoing star formation. Finally, to evaluate systematic uncertainties in the baryon fraction in cosmological simulations we present a comparison of gas fractions in our adiabatic simulations to resimulations of the same objects with the entropy-conserving smoothed particle hydrodynamics (SPH) code Gadget. The cumulative gas fraction profiles in the two sets of simulations on average agree to better than ≈3% outside the cluster core (r/rvir ≳ 0.2) but differ by up to 10% at small radii. The differences are smaller than those found in previous comparisons of Eulerian and SPH simulations. Nevertheless, they are systematic and have to be kept in mind when using gas fractions from cosmological simulations.