  Dark matter subhaloes are key for the predictions of simulations of structure
formation, but their existence frequently ends prematurely due to two technical
issues, namely numerical disruption in N-body simulations and halo finders
failing to identify them. Here we focus on the second issue, using the
phase-space friends-of-friends halo finder ROCKSTAR as a benchmark (though we
expect our results to translate to comparable codes). We confirm that the most
prominent cause for losing track of subhaloes is tidal distortion rather than a
low number of particles. As a solution, we present a flexible post-processing
algorithm that tracks all subhalo particles over time, computes subhalo
positions and masses based on those particles, and progressively removes
stripped matter. If a subhalo is lost by the halo finder, this algorithm keeps
tracking its so-called ghost until it has almost no particles left or has truly
merged with its host. We apply this technique to a large suite of N-body
simulations and restore lost subhaloes to the halo catalogues, which has a
dramatic effect on key summary statistics of large-scale structure.
Specifically, the subhalo mass function increases by about 50% and the halo
correlation function increases by a factor of two at small scales. While these
quantitative results are somewhat specific to our algorithm, they demonstrate
that particle tracking is a promising way to reliably follow haloes and reduce
the need for orphan models. Our algorithm and augmented halo catalogues are
publicly available.
