Ripple patterns in in-plane velocities of OB stars from LAMOST and Gaia
  With about 12 000 OB type stars selected from the LAMOST and Gaia survey, we
study their 3 dimensional velocity distribution over the range of
galactocentric radius from 6 to 15 kpc in the Galactic disk plane. A clear
ripple pattern in the radial velocity ($V_R$) map is shown. The median $V_R$
reaches $-8$ km s$^{-1}$ at $R\sim9$ kpc, then increases to $\sim0$ km s$^{-1}$
at $R\sim12$ kpc, and later declines to below $-10$ km s$^{-1}$ beyond
$R\sim13$ kpc. The median azimuthal velocity ($V_\phi$) map shows a similar
pattern but has roughly $1/4$ phase difference with the radial velocity.
Although the ripple of negative $V_R$ at $\sim9$ kpc extends to about
40$^\circ$ in the azimuth angle, it does not align with either the Local or the
Perseus spiral arm. Moreover, the farther ripple beyond 13 kpc does not match
the Outer spiral arm either. This indicates that the non-axisymmetric kinematic
features are not induced by perturbations of known spiral structures. The
central rotating bar can not lead to such patterns in the outer disk either.
External perturbation of a dwarf galaxy or a dark matter sub-halo can induce
such patterns but requires more evidence from both observations and
simulations. The $V_\phi$ map in the $Z$--$V_Z$ plane of the OB stars is also
investigated. Despite asymmetry to some degree, no spiral pattern is found.
This is reasonable since most of the OB stars have ages much younger than 100
Myrs, which is smaller than one orbital period around the Galactic center.
