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 this same simulation to assess how well are able to recover orbits in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. We find that, for present measurement uncertainties, we are able to recover the apocentre r_a and pericentre r_p to ~ 40%. However, even with better data the non-sphericity of the potential and satellite interactions during group infall make the orbital recovery more challenging. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is consistent with the most massive subhalos that form before z=10, lending support to the idea that the Milky Way dwarfs formed before reionisation.