Tracing the Hercules stream around the Galaxy

TL;DR

This paper models the Hercules stream across the Galactic disk, predicting its velocity distribution and identifying optimal observational directions to test the resonance hypothesis using Gaia and spectroscopic surveys.

Contribution

It extends the resonance model of the Hercules stream beyond the Solar neighborhood, providing predictions for its distribution throughout the Galaxy and identifying promising lines of sight for detection.

Findings

Hercules stream should be detectable in line-of-sight velocities at 250-290 degrees galactic longitude.

Predictions are robust against distance uncertainties and Galactic potential assumptions.

Future surveys can test the resonance origin and constrain the Galactic bar properties.

Abstract

It has been proposed that the Hercules stream, a group of co-moving stars in the Solar neighborhood offset from the bulk of the velocity distribution, is the result of resonant interactions between stars in the outer disk and the Galactic bar. So far it has only been seen in the immediate Solar neighborhood, but the resonance model makes a prediction over a large fraction of the Galactic disk. I predict the distribution of stellar velocities and the changing Hercules feature in this distribution as a function of location in the Galactic disk in a simple model for the Galaxy and the bar that produces the observed Hercules stream. The Hercules feature is expected to be strong enough to be unambiguously detected in the distribution of line-of-sight velocities in selected directions. I identify quantitatively the most promising lines of sight for detection in line-of-sight velocities using the Kullback-Leibler divergence between the predictions of the resonance model and an axisymmetric model; these directions are at 250 deg <~ l <~ 290 deg. The predictions presented here are only weakly affected by distance uncertainties, assumptions about the distribution function in the stellar disk, and the details of the Galactic potential including the effect of spiral structure. Gaia and future spectroscopic surveys of the Galactic disk such as APOGEE and HERMES will be able to robustly test the origin of the Hercules stream and constrain the properties of the Galactic bar.