The GALAH Survey: Stellar streams and how stellar velocity distributions vary with Galactic longitude, hemisphere and metallicity

TL;DR

This study analyzes how stellar velocity structures, especially the Hercules stream, vary with metallicity and viewing direction in the Galaxy, supporting a bar resonant origin and revealing fine local velocity distribution structures.

Contribution

It provides new insights into the dependence of stellar velocity structures on metallicity and galactic viewing angles, supporting a bar resonance model for the Hercules stream.

Findings

Hercules stream is prominent in high-metallicity stars.

The Hercules stream's velocity peak varies with galactic longitude.

Local velocity distributions show fine structure over a few hundred parsecs.

Abstract

Using GALAH survey data of nearby stars, we look at how structure in the planar (u,v) velocity distribution depends on metallicity and on viewing direction within the Galaxy. In nearby stars, with distance d < 1 kpc, the Hercules stream is most strongly seen in higher metallicity stars [Fe/H] > 0.2. The Hercules stream peak v value depends on viewed galactic longitude, which we interpret as due to the gap between the stellar stream and more circular orbits being associated with a specific angular momentum value of about 1640 km/s kpc. The association of the gap with a particular angular momentum value supports a bar resonant model for the Hercules stream. Moving groups previously identified in Hipparcos observations are easiest to see in stars nearer than 250 pc, and their visibility and peak velocities in the velocity distributions depends on both viewing direction (galactic longitude and hemisphere) and metallicity. We infer that there is fine structure in local velocity distributions that varies over distances of a few hundred pc in the Galaxy.