Evidence of a dynamically evolving Galactic warp
  In a cosmological setting, the disc of a galaxy is expected to continuously
experience gravitational torques and perturbations from a variety of sources,
which can cause the disc to wobble, flare and warp. Specifically, the study of
galactic warps and their dynamical nature can potentially reveal key
information on the formation history of galaxies and the mass distribution of
their halos. Our Milky Way presents a unique case study for galactic warps,
thanks to the detailed knowledge of its stellar distribution and kinematics.
Using a simple model of how the warp's orientation is changing with time, we
here measure the precession rate of the Milky Way's warp using 12 million giant
stars from Gaia Data Release 2, finding that it is precessing at $10.86 \pm
0.03_{stat} \pm 3.20_{sys}$ km/s/kpc in the direction of Galactic rotation,
about one third the angular velocity at the Sun's position in the Galaxy. The
direction and magnitude of the warp's precession rate favour the scenario that
the warp is the results of a recent or ongoing encounter with a satellite
galaxy, rather than the relic of the ancient assembly history of the Galaxy.
