The Shape of the Milky Way Halo

The latest Sloan Digital Sky Survey data tracing the tidal stream from the Sagittarius dwarf spheroidal (dSph) reveals a bifurcation in the distribution of debris. The branching is caused by the projection of the old trailing and young leading tidal arms of the Sagittarius dSph. The angular difference between the branches is a measure of the precession of the orbital plane and hence the asphericity of the potential. Using numerical simulations, we find that the existence of the bifurcation is a strong constraint on the shape of the halo, which has to be close to spherical. This result is not sensitive to variations in the proper motion of Sagittarius (within the observationally permitted range) and has been checked against changes in the details of the Milky Way potential