The Galactic warp revealed by Gaia DR2 kinematics
  Using Gaia DR2 astrometry, we map the kinematic signature of the Galactic
stellar warp out to a distance of 7 kpc from the Sun. Combining Gaia DR2 and
2MASS photometry, we identify, via a probabilistic approach, 599 494 upper main
sequence stars and 12 616 068 giants without the need for individual extinction
estimates. The spatial distribution of the upper main sequence stars clearly
shows segments of the nearest spiral arms. The large-scale kinematics of both
the upper main sequence and giant populations show a clear signature of the
warp of the Milky Way, apparent as a gradient of 5-6 km/s in the vertical
velocities from 8 to 14 kpc in Galactic radius. The presence of the signal in
both samples, which have different typical ages, suggests that the warp is a
gravitationally induced phenomenon.
