On the behaviour of streams in angle and frequency spaces in different
  potentials
  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.
