Extending the halo mass resolution of N-body simulations

We present a scheme to extend the halo mass resolution of dark matter N-body simulations. The method uses the simulated density field to predict the number of sub-resolution haloes expected in different regions, taking as input the abundance and the bias factors of haloes of a given mass. These quantities can be computed analytically or measured from higher resolution simulations. We show that the method recovers the abundance and clustering in real- and redshift-space of haloes with mass below ∼7.5 × 1013 h−1 M⊙ at z = 0 to better than 10 per cent. By applying the method to an ensemble of 50 low-resolution, large-volume simulations, we compute the expected correlation function and covariance matrix of luminous red galaxies (LRGs), which we compare to state-of-the-art baryonic acoustic oscillation measurements. The original simulations resolve just two-thirds of the LRG population, so we extend their resolution by a factor of 30 in halo mass in order to recover all LRGs. Using our method, it is now feasible to build the large numbers of high-resolution large volume mock galaxy catalogues required to compute the covariance matrices necessary to analyse upcoming galaxy surveys designed to probe dark energy.