Galactic warp from the kinematics of OB stars.

The warp in Milky Way seen through the young stellar disk shows the complex structures. Its spatial distribution reveals the asymmetry in the vertical distance to the galactic midplane, whereas the kinematics shows the os- cillating motion. We analyze the warp using about 25,000 OB stars from Gaia Data Release 2, with Galactocentric azimuth (φ) range from 135^◦ to 225^◦. We detect the warp from two distinctive regions. First region (warp down) lies on 100^◦ < l < 150^◦ and second regions (warp up) lies on 150^◦ < l < 220^◦. The vertical position towards the Galactic midplane (Z) from all samples shows the line-of-nodes of the warp is on φ = 174◦ ± 2◦. The maximum deviation in warp down region reachs 0.1 kpc and 0.2 kpc in warp up region. The vertical velocity (V_Z) from both regions are dominated by negative V_Z. From the kinematics dis- tribution, it can be concluded that there is an oscillati1ng motion. Considering with the previous study, the oscillating motion was caused by the gravitational interaction with the Galaxy satellites. From this, we can constrain the origin of the warp is developing from the tidal interaction between Milky Way and its satellites. [ABSTRACT FROM AUTHOR]