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 .