Streaming Motions and Kinematic Distances to Molecular Clouds
  We present high-resolution smoothed particle hydrodynamics simulations of a
region of gas flowing in a spiral arm and identify dense gas clouds to
investigate their kinematics with respect to a Milky Way model. We find that,
on average, the gas in the arms can have a net radial streaming motion of $v_R
\approx -9 \,\mathrm{km/s}$ and rotate $\approx 6 \,\mathrm{km/s}$ slower than
the circular velocity. This translates to average peculiar motions towards the
Galaxy centre and opposite to Galactic rotation. These results may be sensitive
to the assumed spiral arm perturbation, which is $\approx 3\%$ of the disc
potential in our model. We compare the actual distance and the kinematic
estimate and we find that streaming motions introduce systematic offsets of
$\approx 1$ kpc. We find that the distance error can be as large as $\pm 2$ kpc
and the recovered cloud positions have distributions that can extend
significantly into the inter-arm regions. We conclude that this poses a
difficulty in tracing spiral arm structure in molecular cloud surveys.
