3D Asymmetrical motions of the Galactic outer disk with LAMOST K giant
  stars
  We present a three dimensional velocity analysis of Milky Way disk kinematics
using LAMOST K giant stars and the GPS1 proper motion catalogue. We find that
Galactic disk stars near the anticenter direction (in the range of
Galactocentric distance between $R=8$ and $13$ kpc and vertical position
between $Z=-2$ and $2$ kpc) exhibit asymmetrical motions in the Galactocentric
radial, azimuthal, and vertical components. Radial motions are not zero, thus
departing from circularity in the orbits; they increase outwards within
$R\lesssim 12$ kpc, show some oscillation in the northern ($0 < Z < 2$ kpc)
stars, and have north-south asymmetry in the region corresponding to a
well-known nearby northern structure in the velocity field. There is a clear
vertical gradient in azimuthal velocity, and also an asymmetry that shifts from
a larger azimuthal velocity above the plane near the solar radius to faster
rotation below the plane at radii of 11-12 kpc. Stars both above and below the
plane at $R\gtrsim 9$ kpc exhibit net upward vertical motions. We discuss some
possible mechanisms that might create the asymmetrical motions, such as
external perturbations due to dwarf galaxy minor mergers or dark matter
sub-halos, warp dynamics, internal processes due to spiral arms or the Galactic
bar, and (most likely) a combination of some or all of these components.
