Tracing Hercules in Galactic azimuth with Gaia DR2
  The second data release of the Gaia mission has revealed, in stellar velocity
and action space, multiple ridges, the exact origin of which is still debated.
Recently, we demonstrated that a large Galactic bar with pattern speed 39
km/s/kpc does create most of the observed ridges. Among those ridges, the
Hercules moving group would then be associated to orbits trapped at the
co-rotation resonance of the bar. Here we show that a distinctive prediction of
such a model is that the angular momentum of Hercules at the Sun's radius must
significantly decrease with increasing Galactocentric azimuth, i.e. when
getting closer to the major axis of the bar. We show that such a dependence of
the angular momentum of trapped orbits on the azimuth would on the other hand
not happen close to the outer Lindblad resonance of a faster bar, unless the
orbital distribution is still far from phase-mixed, namely for a bar
perturbation younger than ~ 2 Gyr. Using Gaia DR2 and Bayesian distances from
the StarHorse code, and tracing the average Galactocentric radial velocity as a
function of angular momentum and azimuth, we show that the Hercules angular
momentum changes significantly with azimuth as expected for the co-rotation
resonance of a dynamically old large bar.
