2019ApJ...872L...1C

With about 12,000 OB type stars selected from the LAMOST and Gaia survey, we study their three-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 ∼ 9 kpc, then increases to ∼0 km s^–1 at R ∼ 12 kpc, and later declines to below -10 km s^–1 beyond R ∼ 13 kpc. The median azimuthal velocity (V_ph) map shows a similar pattern but has roughly one-quarter phase difference with the radial velocity. Although the ripple of negative V_R at ∼9 kpc extends to about 40^* in the azimuth angle, it does not align with either the Local or the Perseus spiral arms. 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 cannot 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 this requires more evidence from both observations and simulations. The V_ph 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 as most of the OB stars have ages that are much younger than 100 Myr, which is smaller than one orbital period around the Galactic center.