Kinematics with Gaia DR2: The Force of a Dwarf
  We use Gaia DR2 astrometric and line-of-sight velocity information combined
with two sets of distances obtained with a Bayesian inference method to study
the 3D velocity distribution in the Milky Way disc. We search for variations in
all Galactocentric cylindrical velocity components ($V_{\phi}$, $V_R$ and
$V_z$) with Galactic radius, azimuth, and distance from the disc mid-plane. We
confirm recent work showing that bulk vertical motions in the $R\text{-}z$
plane are consistent with a combination of breathing and bending modes. In the
$x\text{-}y$ plane, we show that, although the amplitudes change, the structure
produced by these modes is mostly invariant as a function of distance from the
plane. Comparing to two different Galactic disc models, we demonstrate that the
observed patterns can drastically change in short time intervals, showing the
complexity of understanding the origin of vertical perturbations. A strong
radial $V_R$ gradient was identified in the inner disc, transitioning smoothly
from $16$ km s$^{-1}$ kpc$^{-1}$ at an azimuth of $30^\circ<\phi<45^\circ$
ahead of the Sun-Galactic centre line, to $-16$ km s$^{-1}$ kpc$^{-1}$ at an
azimuth of $-45^\circ<\phi<-30^\circ$ lagging the solar azimuth. We use a
simulation with no significant recent mergers to show that exactly the opposite
trend is expected from a barred potential, but overestimated distances can flip
this trend to match the data. Alternatively, using an $N$-body simulation of
the Sagittarius dwarf-Milky Way interaction, we demonstrate that a major recent
perturbation is necessary to reproduce the observations. Such an impact may
have strongly perturbed the existing bar or even triggered its formation in the
last $1\text{-}2$ Gyr.
