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We present and test a code for two-fluid simulations of galaxy formation, one of the fluids being collision-less. The hydrodynamical evolution is solved through the SPH method while gravitational forces are calculated using a tree method. The code is Lagrangian, and fully adaptive both in space and time. A significant fraction gas in simulations of hierarchical galaxy formation ends up in tight clumps where it is, in terms of computational effort, very expensive to integrate the SPH equations. Furthermore, this is a computational waste since these tight gas clumps are typically not gravitationally resolved. We solve this by merging gas particles in these regions, thereby limiting the SPH resolution to the gravitational force resolution, and further speeding up the simulation through the reduction in the number of particles., Comment: 15 pages, 16 figures

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., Comment: 12 pages, 21 figures. Astronomy and Astrophysics (final manuscript submitted)