Tracing the Hercules stream with Gaia and LAMOST: new evidence for a
  fast bar in the Milky Way
  The length and pattern speed of the Milky Way bar are still controversial.
Photometric and spectroscopic surveys of the inner Galaxy, as well as gas
kinematics, favour a long and slowly rotating bar, with corotation around a
Galactocentric radius of 6 kpc. On the other hand, the existence of the
Hercules stream in local velocity space favours a short and fast bar with
corotation around 4 kpc. This follows from the fact that the Hercules stream
looks like a typical signature of the outer Lindblad resonance of the bar. As
we showed recently, reconciling this local stream with a slow bar would need to
find a yet unknown alternative explanation, based for instance on the effect of
spiral arms. Here, by combining the TGAS catalogue of the Gaia DR1 with LAMOST
radial velocities, we show that the position of Hercules in velocity space as a
function of radius in the outer Galaxy indeed varies exactly as predicted by
fast bar models with a pattern speed no less than 1.8 times the circular
frequency at the Sun's position.
