A comparison of gas dynamics in smooth particle hydrodynamics and semi-analytic models of galaxy formation

We compare the results of two techniques used to calculate the evolution of cooling gas during galaxy formation: smooth particle hydrodynamics (SPH) simulations and semi-analytic modelling. We improve upon the earlier statistical comparison of Benson et al. by taking halo merger histories from the dark matter component of the SPH simulation, which allows us to compare the evolution of galaxies on an object-by-object basis in the two treatments. We use a ‘stripped-down’ version of the semi-analytic model described by Helly et al. that includes only shock heating and radiative cooling of gas and which is adjusted to mimic the resolution and other parameters of a comparison SPH simulation as closely as possible. We compare the total mass of gas that cools in haloes of different mass as a function of redshift and the masses and spatial distribution of individual ‘galaxies’. At a redshift of z = 0, the cooled gas mass in well-resolved haloes agrees remarkably well (to better than ∼20 per cent) in the SPH simulation and stripped-down semi-analytic model. At high redshift, resolution effects in the simulation become increasingly important and, as a result, more gas tends to cool in low-mass haloes in the SPH simulation than in the semi-analytic model. The cold gas mass function of individual galaxies in the two treatments at z = 0 also agrees very well and, when the effects of mergers are accounted for, the masses of individual galaxies and their two-point correlation functions are also in excellent agreement in the two treatments. Thus, our comparison confirms and extends the earlier conclusion of Benson et al. that SPH simulations and semi-analytic models give consistent results for the evolution of cooling galactic gas.