We present a new method for determining the Galactic gravitational potential
based on forward modeling of tidal stellar streams. We use this method to test
the performance of smooth and static analytic potentials in representing
realistic dark matter halos, which have substructure and are continually
evolving by accretion. Our FAST-FORWARD method uses a Markov Chain Monte Carlo
algorithm to compare, in 6D phase space, an "observed" stream to models created
in trial analytic potentials. We analyze a large sample of streams evolved in
the Via Lactea II (VL2) simulation, which represents a realistic Galactic halo
potential. The recovered potential parameters are in agreement with the best
fit to the global, present-day VL2 potential. However, merely assuming an
analytic potential limits the dark matter halo mass measurement to an accuracy
of 5 to 20%, depending on the choice of analytic parametrization. Collectively,
mass estimates using streams from our sample reach this fundamental limit, but
individually they can be highly biased. Individual streams can both under- and
overestimate the mass, and the bias is progressively worse for those with
smaller perigalacticons, motivating the search for tidal streams at
galactocentric distances larger than 70 kpc. We estimate that the assumption of
a static and smooth dark matter potential in modeling of the GD-1 and Pal5-like
streams introduces an error of up to 50% in the Milky Way mass estimates.Comment: 12 pages, 6 figures, submitted to ApJ; more information on our stream
sample and a movie of the potential recovery method used can be found at
http://www.astro.yale.edu/abonaca/research/potential_recovery.htm
