Behaviour of streams in angle and frequency spaces in different potentials

Abstract

We have studied the behaviour of stellar streams in the Aquarius fully cosmological N-body simulations of the formation of Milky Way halos. In particular, we have characterised the streams in angle/frequency spaces derived using an approximate but generally well-fitting spherical potential. We have also run several test-particle simulations to understand and guide our interpretation of the different features we see in the Aquarius streams. Our goal is both to establish which deviations of the expected action-angle behaviour of streams exist because of the approximations made on the potential, but also to derive to what degree we can use these coordinates to model streams reliably. We have found that many of the Aquarius streams wrap in angle space along relatively straight lines, and also in frequency space. On the other hand, from our controlled simulations we have been able to establish that deviations from spherical symmetry, the use of incorrect potentials and the inclusion of self-gravity lead to streams in angle space to still be along relatively straight lines but also to depict wiggly behaviour whose amplitude increases as the approximation to the true potential becomes worse. In frequency space streams typically become thicker and somewhat distorted. Therefore, our analysis explains most of the features seen in the approximate angle and frequency spaces for the Aquarius streams with the exception of their somewhat `noisy' and `patchy' morphologies. These are likely due to the interactions with the large number of dark matter subhalos present in the cosmological simulations. Since the measured angle-frequency misalignments of the Aquarius streams can largely be attributed to using the wrong (spherical) potential, determining the mass growth history of these halos will only be feasible once the true potential has been determined robustly.