Mapping the Galactic disk with the LAMOST and Gaia Red clump sample:
  III: A new velocity substructure and time stamps of the Galactic disk
  asymmetry in the disk between 12$-$15 \,kpc
  We investigate the three-dimensional asymmetrical kinematics and present time
stamps of the Milky Way disk between Galactocentric distances of $R=12$ and 15
\,kpc, using red clump stars selected from the LAMOST Galactic survey, also
with proper motion measurements provided by the Gaia DR2. We discover velocity
substructure above the Galactic plane corresponding to a density dip found
recently ("south-middle opposite" density structure[R $\sim$ 12-15 \,kpc, Z
$\sim$ 1.5 \,kpc] discovered in \citet{Wang2018b, Wang2018c}) in the radial and
azimuthal velocity. For the vertical velocity, we detect clear vertical bulk
motions or bending mode motions, which has no clear north-south asymmetry
corresponding to the in-plane asymmetrical features. In the subsample of stars
with different ages, we find that there is little temporal evolution of the
in-plane asymmetry from 0$-$14 \,Gyr, which means the structure is sensitive to
the perturbations in almost cosmic time possibly. We propose that the possible
scenario of this asymmetric velocity structure is caused by the mechanisms
generated in-plane, rather than vertical perturbations.
