The shape of the inner Milky Way halo from observations of the Pal 5 and GD-1 stellar streams

Understand

We constrain the shape of the Milky Way's halo by dynamical modeling of the observed phase-space tracks of the Pal 5 and GD-1 tidal streams.

• We find that the only information about the potential gleaned from the tracks of these streams are precise measurements of the shape of the gravitational potential---the ratio of vertical to radial acceleration---at the location of the streams, with weaker constraints on the radial and vertical accelerations separately.
• The latter will improve significantly with precise proper-motion measurements from Gaia.
• We measure that the overall potential flattening is 0.95 +/- 0.04 at the location of GD-1 ([R,z] ~ [12.5,6.7] kpc) and 0.94 +/- 0.05 at the position of Pal 5 ([R,z] ~ [8.4,16.8] kpc).