Transient spiral structure and the disc velocity substructure in Gaia
  DR2
  The second data release from ESA's Gaia mission has revealed many ridge-like
structures in the velocity distribution of the Milky Way. We show that these
can arise naturally from winding transient spiral structure that is commonly
seen in N-body simulations of disk galaxies. We construct test particle models
of the winding spiral structure, and compare the resulting distribution of
orbits with the observed two-dimensional velocity distribution in the extended
solar neighbourhood and with the distribution of rotational velocities over 8
kpc along the Sun--Galactic-centre--Galactic anti-centre line. We show that the
ridges in these observations are well reproduced by the winding spiral model.
Additionally, we demonstrate that the transient winding spiral potential can
create a Hercules-like feature in the kinematics of the solar neighbourhood,
either alone, or in combination with a long-slow bar potential.
