The signatures of the resonances of a large Galactic bar in local
  velocity space
  The second data release of the Gaia mission has revealed a very rich
structure in local velocity space. In terms of in-plane motions, this rich
structure is also seen as multiple ridges in the actions of the axisymmetric
background potential of the Galaxy. These ridges are probably related to a
combination of effects from ongoing phase-mixing and resonances from the spiral
arms and the bar. We have recently developed a method to capture the behaviour
of the stellar phase-space distribution function at a resonance, by
re-expressing it in terms of a new set of canonical actions and angles
variables valid in the resonant region. Here, by properly treating the
distribution function at resonances, and by using a realistic model for a
slowly rotating large Galactic bar with pattern speed 39 km/s/kpc, we show that
no less than six ridges in local action space can be related to resonances with
the bar. Two of these at low angular momentum correspond to the corotation
resonance, and can be associated to the Hercules moving group in local velocity
space. Another one at high angular momentum corresponds to the outer Lindblad
resonance, and can tentatively be associated to the velocity structure seen as
an arch at high azimuthal velocities in Gaia data. The other ridges are
associated to the 3:1, 4:1 and 6:1 resonances. The latter can be associated to
the so-called 'horn' of the local velocity distribution. While it is clear that
effects from spiral arms and incomplete phase-mixing related to external
perturbations also play a role in shaping the complex kinematics revealed by
Gaia data, the present work demonstrates that, contrary to common
misconceptions, the bar alone can create multiple prominent ridges in velocity
and action space.
