Probing the Galaxy's bars via the Hercules stream

TL;DR

This study investigates how the Galactic's main and long bars influence local stellar velocities, revealing their pattern speeds and identifying a new kinematic feature possibly linked to the Arcturus stream.

Contribution

It provides the first detailed analysis of the combined effects of both the Galactic bar and long bar on local stellar kinematics, including pattern speed measurements and a new velocity feature.

Findings

Both bars can produce Hercules-like features.

Measured pattern speeds of 1.87 +/- 0.02 times the local circular frequency.

Identified a new kinematic feature possibly related to the Arcturus stream.

Abstract

It has been suggested that a resonance between a rotating bar and stars in the solar neighbourhood can produce the so called 'Hercules stream'. Recently, a second bar may have been identified in the Galactic centre, the so called 'long bar', which is longer and much flatter than the traditional Galactic bar, and has a similar mass. We looked at the dynamical effects of both bars, separately and together, on orbits of stars integrated backwards from local position and velocities, and a model of the Galactic potential which includes the bars directly. Both bars can produce Hercules like features, and allow us to measure the rotation rate of the bar(s). We measure a pattern speed, for both bars, of 1.87 +/- 0.02 times the local circular frequency. This is on par with previous measurements for the Galactic bar, although we do adopt a slightly different Solar motion. Finally, we identify a new kinematic feature in local velocity space, caused by the long bar, which is tempting to identify with the high velocity 'Arcturus' stream.