A Comparison of Semi-Analytic and Smoothed Particle Hydrodynamics Galaxy Formation

We compare the statistical properties of bright galaxies found in two different models of galaxy formation — the semi-analytic model of Cole et al. (1999) and the smoothed particle hydrodynamics (SPH) simulations of Pearce et al. (1999) — in cold dark matter universes. These two techniques model gas processes very differently: by approximate, analytic methods in the case of semi-analytic models, and by direct numerical integration of the equations of hydrodynamics for a set of discrete particles in the case of SPH. Using a ‘stripped-down’ version of the semi-analytic model which mimics the resolution of the SPH simulations and excludes physical processes not included in the simulations, we find that the two models produce an ensemble of galaxies with remarkably similar properties. The full semi-analytic model, which has effectively no resolution limit and which includes the important astrophysical processes of star formation and supernovae feedback, produces somewhat different (but readily understandable) results. Our comparison demonstrates that, on the whole, SPH simulations and semi-analytic models give similar results for the thermodynamic evolution of cooling gas in cosmological volumes. Agreement is particularly good for the global fractions of hot, cold and uncollapsed gas, and for the total amount of cooled gas in halos of different mass, for which the SPH and stripped-down semi-analytic calculations differ by at most 50%. The abundance of galactic-size objects as a function of cold gas mass agrees to within a factor of 2, and the correlation functions agree to better than about 50% over most of their range of pair separations. Our comparison demonstrates that these different techniques for modelling galaxy formation produce results that are broadly consistent with each other.