2016ApJ...833...31B

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). Combined with constraints on the force field near the Galactic disk, we determine that the axis ratio of the dark-matter halo's density distribution is 1.05 ±0.14 within the inner 20 kpc, with a hint that the halo becomes more flattened near the edge of this volume. The halo mass within 20 kpc is (1.1±0.1)×10¹¹ M_☉. A dark-matter halo this close to spherical is in tension with the predictions from numerical simulations of the formation of dark-matter halos.