Gaia-DR2 extended kinematical maps. Part II: Dynamics in the Galactic
  disk explaining radial and vertical velocities
  Context: In our Paper I, by using statistical deconvolution methods, extended
kinematics maps of Gaia-DR2 data have been produced in a range of heliocentric
distances that are a factor of two to three larger than those analyzed
previously by the Gaia Collaboration with the same data. It added the range of
Galactocentric distances between 13 kpc and 20 kpc to the previous maps.
  Aims: Here, we investigate the dynamical effects produced by different
mechanisms that can explain the radial and vertical components of these
extended kinematic maps, including a decomposition of bending and breathing of
the vertical components. This paper as a whole tries to be a compendium of
different dynamical mechanisms whose predictions can be compared to the
kinematic maps.
  Methods: Using analytical methods or simulations, we are able to predict the
main dynamical factors and compare them to the predictions of the extended
kinematic maps of Gaia-DR2.
  Results: The gravitational influence of Galactic components that are
different from the disk, such as the long bar or bulge, the spiral arms, or a
tidal interaction with Sagittarius dwarf galaxy, may explain some features of
the velocity maps, especially in the inner parts of the disk. However, they are
not sufficient in explaining the most conspicuous gradients in the outer disk.
Vertical motions might be dominated by external perturbations or mergers,
although a minor component may be due to a warp whose amplitude evolves with
time. Here, we show with two different methods, which analyze the dispersion of
velocities, that the mass distribution of the disk is flared. Despite these
partial explanations, the main observed features can only be explained in terms
of out-of-equilibrium models, which are either due to external perturbers or to
the fact that the disk has not had time to reach equilibrium since its
formation.
