Action-based Dynamical Modeling for the Milky Way Disk: The Influence of
  Spiral Arms
  RoadMapping is a dynamical modeling machinery developed to constrain the
Milky Way's (MW) gravitational potential by simultaneously fitting an
axisymmetric parametrized potential and an action-based orbit distribution
function (DF) to discrete 6D phase-space measurements of stars in the Galactic
disk. In this work we demonstrate RoadMapping's robustness in the presence of
spiral arms by modeling data drawn from an N-body simulation snapshot of a
disk-dominated galaxy of MW mass with strong spiral arms (but no bar),
exploring survey volumes with radii 500pc<=r_max<=5kpc. The potential
constraints are very robust, even though we use a simple action-based DF, the
quasi-isothermal DF (qDF). The best-fit RoadMapping model always recovers the
correct gravitational forces where most of the stars that entered the analysis
are located, even for small volumes. For data from large survey volumes,
RoadMapping finds axisymmetric models that average well over the spiral arms.
Unsurprisingly, the models are slightly biased by the excess of stars in the
spiral arms. Gravitational potential models derived from survey volumes with at
least r_max=3kpc can be reliably extrapolated to larger volumes. However, a
large radial survey extent, r_max~5kpc, is needed to correctly recover the halo
scale length. In general, the recovery and extrapolability of potentials
inferred from data sets which were drawn from inter-arm regions appear to be
better than those of data sets drawn from spiral arms. Our analysis implies
that building axisymmetric models for the Galaxy with upcoming Gaia data will
lead to sensible and robust approximations of the MW's potential.
