Paper detail

Merger rates in hierarchical models of galaxy formation. II: Comparison with N-body simulations

We have made a detailed comparison of the results of large N-body simulations with the analytical description of the merging histories of dark matter halos presented in Lacey & Cole 1993, which is based on an extension of the Press- Schechter method (Bond etal 1991,Bower 1991). We find the analytical predictions for the halo mass function, merger rates and formation times to be remarkably accurate. The analytical model is however expected to apply for arbitrary Omega and general power spectra. Dark matter halos were identified in the simulations using two different methods. For halos selected at mean overdensities 100-200, the analytical mass function was found to provide a good fit to the simulations with a collapse threshold close to that predicted by the spherical collapse model, with a typical error of <30% over a range of 1000 in mass, which is the full dynamic range of our N-body simulations. This was insensitive to the type of filtering used. Over a range of 100 - 1000 in mass, there was also good agreement with the analytical predictions for merger rates, including their dependence on the masses of the two halos involved and the time interval being considered, and for formation times, including the dependence on halo mass and formation epoch. The analytical Press-Schechter mass function and its extension to halo lifetimes and merger rates thus provide a very useful description of the growth of dark matter halos through hierarchical clustering, and should providea valuable tool in studies of the formation and evolution of galaxies and galaxy clusters.