Determining orbits for the Milky Way's dwarfs
  We calculate orbits for the Milky Way dwarf galaxies with proper motions, and
compare these to subhalo orbits in a high resolution cosmological simulation.
We use the simulation data to assess how well orbits may be recovered in the
face of measurement errors, a time varying triaxial gravitational potential,
and satellite-satellite interactions. For present measurement uncertainties, we
recover the apocentre r_a and pericentre r_p to ~40%. With improved data from
the Gaia satellite we should be able to recover r_a and r_p to ~14%,
respectively. However, recovering the 3D positions and orbital phase of
satellites over several orbits is more challenging. This owes primarily to the
non-sphericity of the potential and satellite interactions during group infall.
Dynamical friction, satellite mass loss and the mass evolution of the main halo
play a more minor role in the uncertainties.
  We apply our technique to nine Milky Way dwarfs with observed proper motions.
We show that their mean apocentre is lower than the mean of the most massive
subhalos in our cosmological simulation, but consistent with the most massive
subhalos that form before z=10. This lends further support to the idea that the
Milky Way's dwarfs formed before reionisation.
