Gaia Data Release 2: Mapping the Milky Way disc kinematics
  To illustrate the potential of GDR2, we provide a first look at the
kinematics of the Milky Way disc, within a radius of several kiloparsecs around
the Sun. We benefit for the first time from a sample of 6.4 million F-G-K stars
with full 6D phase-space coordinates, precise parallaxes, and precise Galactic
cylindrical velocities . From this sample, we extracted a sub-sample of 3.2
million giant stars to map the velocity field of the Galactic disc from
$\sim$5~kpc to $\sim$13~kpc from the Galactic centre and up to 2~kpc above and
below the plane. We also study the distribution of 0.3 million solar
neighbourhood stars ($r < 200$~pc), with median velocity uncertainties of
0.4~km/s, in velocity space and use the full sample to examine how the
over-densities evolve in more distant regions. GDR2 allows us to draw 3D maps
of the Galactocentric median velocities and velocity dispersions with
unprecedented accuracy, precision, and spatial resolution. The maps show the
complexity and richness of the velocity field of the galactic disc. We observe
streaming motions in all the components of the velocities as well as patterns
in the velocity dispersions. For example, we confirm the previously reported
negative and positive galactocentric radial velocity gradients in the inner and
outer disc, respectively. Here, we see them as part of a non-axisymmetric
kinematic oscillation, and we map its azimuthal and vertical behaviour. We also
witness a new global arrangement of stars in the velocity plane of the solar
neighbourhood and in distant regions in which stars are organised in thin
substructures with the shape of circular arches that are oriented approximately
along the horizontal direction in the $U-V$ plane. Moreover, in distant
regions, we see variations in the velocity substructures more clearly than ever
before, in particular, variations in the velocity of the Hercules stream.
(abridged)
