Inferring the gravitational potential of the Milky Way with a few
  precisely measured stars
  The dark matter halo of the Milky Way is expected to be triaxial and filled
with substructure. It is hoped that streams or shells of stars produced by
tidal disruption of stellar systems will provide precise measures of the
gravitational potential to test these predictions. We develop a method for
inferring the Galactic potential with tidal streams based on the idea that the
stream stars were once close in phase space. Our method can flexibly adapt to
any form for the Galactic potential: it works in phase-space rather than
action-space and hence relies neither on our ability to derive actions nor on
the integrability of the potential. Our model is probabilistic, with a
likelihood function and priors on the parameters. The method can properly
account for finite observational uncertainties and missing data dimensions. We
test our method on synthetic datasets generated from N-body simulations of
satellite disruption in a static, multi-component Milky Way including a
triaxial dark matter halo with observational uncertainties chosen to mimic
current and near-future surveys of various stars. We find that with just four
well-measured stream stars, we can infer properties of a triaxial potential
with precisions of order 5-7 percent. Without proper motions we obtain 15
percent constraints on potential parameters and precisions around 25 percent
for recovering missing phase-space coordinates. These results are encouraging
for the eventual goal of using flexible, time-dependent potential models
combined with larger data sets to unravel the detailed shape of the dark matter
distribution around the Milky Way.
