Cooling Limits on Galaxy Formation; Gas dynamical simulations incorporating a background UV field and metal enrichment

Abstract

We present hydrodynamical simulations of the formation of galaxies in the mass range 10^9 - 10^13 solar masses, with the focus on the efficiency of gas cooling. The effect of a background UV radiation field, and the effect of metal enrichment of halo gas due to star formation and stellar evolution, are investigated. A background radiation field is found to strongly suppress the formation of galaxies with circular velocities less than approximately 50 km/s. The effect is, however, not large enough to reconcile hierarchical clustering models with observations. Metal enrichment of the halo gas increases the cooling rate at low redshifts. We find that the mass fraction of gas at virial temperatures may be reduced by a factor of two, in simulations with a UV background field added. The decrease in overall efficiency of gas cooling due to the inclusion of a UV background field can be more than compensated for by the increased cooling that follows metal enrichment of halo gas, but the effect may depend strongly on the assumed model of metal enrichment.