Tracing the Hercules stream with Gaia and LAMOST: new evidence for a fast bar in the Milky Way

TL;DR

This study uses Gaia and LAMOST data to provide evidence supporting a fast, short Milky Way bar by analyzing the Hercules stream's behavior across different galactic radii.

Contribution

It offers new observational evidence linking the Hercules stream's properties to a fast, short Galactic bar, resolving previous conflicting models.

Findings

Hercules stream position varies with radius as predicted by fast bar models.

Data supports a pattern speed at least 1.8 times the local circular frequency.

Results favor a short, fast rotating Galactic bar over a long, slow one.

Abstract

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.