Streams going Notts: the tidal debris finder comparison project

While various codes exist to systematically and robustly find haloes and subhaloes in cosmological simulations, this is the first work to introduce and rigorously test codes that find tidal debris (streams and other unbound substructure) in fully cosmological simulations of structure formation. We use one tracking and three non-tracking codes to identify substructure (bound and unbound) in a Milky Way type simulation from the Aquarius suite and post-process their output with a common pipeline to determine the properties of these substructures in a uniform way. By using output from a fully cosmological simulation, we also take a step beyond previous studies of tidal debris that have used simple toy models. We find that both tracking and non-tracking codes agree well on the identification of subhaloes and more importantly, the unbound tidal features associated with them. The distributions of basic properties of the total substructure distribution (mass, velocity dispersion, position) are recovered with a scatter of similar to 20 per cent. Using the tracking code as our reference, we show that the non-tracking codes identify complex tidal debris with purities of similar to 40 per cent. Analysing the results of the substructure finders, we find that the general distribution of substructures differ significantly from the distribution of bound subhaloes. Most importantly, both bound and unbound substructures together constitute similar to 18 per cent of the host halo mass, which is a factor of similar to 2 higher than the fraction in self-bound subhaloes. However, this result is restricted by the remaining challenge to cleanly define when an unbound structure has become part of the host halo. Nevertheless, the more general substructure distribution provides a more complete picture of a halo's accretion history.