The Hercules stream as seen by APOGEE-2 South

TL;DR

This study detects the Hercules stream in the APOGEE-2 South data, supporting the hypothesis that it results from the outer Lindblad resonance of a fast Galactic bar, with implications for understanding the Milky Way's dynamics.

Contribution

First detection of the Hercules stream in the APOGEE-2 South data along a specific line of sight, providing evidence favoring the fast-bar (OLR) model over the slow-bar (corotation) model.

Findings

Hercules-like feature detected up to 4 kpc from the Sun.

Data matches well with the fast-bar (OLR) model predictions.

Slow-bar (corotation) model provides a poorer fit to the observed data.

Abstract

The Hercules stream is a group of co-moving stars in the Solar neighbourhood, which can potentially be explained as a signature of either the outer Lindblad resonance (OLR) of a fast Galactic bar or the corotation resonance of a slower bar. In either case, the feature should be present over a large area of the disc. With the recent commissioning of the APOGEE-2 Southern spectrograph we can search for the Hercules stream at $(l,b)=(270^\circ,0)$, a direction in which the Hercules stream, if caused by the bar's OLR, would be strong enough to be detected using only the line-of-sight velocities. We clearly detect a narrow, Hercules-like feature in the data that can be traced from the solar neighbourhood to a distance of about 4 kpc. The detected feature matches well the line-of-sight velocity distribution from the fast-bar (OLR) model. Confronting the data with a model where the Hercules stream is caused by the corotation resonance of a slower bar leads to a poorer match, as the corotation model does not predict clearly separated modes, possibly because the slow-bar model is too hot.