[Paper Review] The GALAH Survey Sees Stellar Streams: Dependence of Nearby Velocity Distributions on Galactic Longitude and Metallicity

Alice C. Quillen, Gayandhi De Silva+31 more|arXiv (Cornell University)|Feb 8, 2018

Stellar, planetary, and galactic studies3 citations

TL;DR

Using GALAH survey data on nearby stars (d ≤ 1 kpc), this study reveals that the Hercules stellar stream's velocity peak depends on galactic longitude and metallicity, with stronger signals in [Fe/H] > 0.2 stars. The observed dependence on viewing direction and angular momentum (≈1620 km/s kpc) supports a bar resonant origin for the stream, indicating fine-structured velocity substructure within 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 $d \lesssim 1$ 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 1620 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 closer stars with $d<250$ 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.

Motivation & Objective

• To investigate how stellar velocity distributions in the Galactic disk vary with galactic longitude and metallicity using nearby stars.
• To determine whether observed kinematic substructures, such as the Hercules stream, are influenced by viewing direction and metallicity.
• To test whether the dependence of velocity peaks on galactic longitude can be explained by a bar resonant model.
• To assess the visibility and kinematic properties of moving groups in different distance and metallicity bins.

Proposed method

• Analysis of radial velocity and proper motion data from the GALAH survey for stars within d ≤ 1 kpc.
• Construction of planar (u,v) velocity distributions binned by galactic longitude and metallicity [Fe/H].
• Identification of kinematic substructures, particularly the Hercules stream, in velocity space across different viewing directions.
• Correlation of velocity peak positions with angular momentum values to test for resonant structures.
• Comparison of results with Hipparcos-based moving group detections to assess distance and metallicity dependence.
• Use of angular momentum thresholds (≈1620 km/s kpc) to interpret the gap between streams and circular orbits as a resonant signature.

Experimental results

Research questions

• RQ1 How does the visibility of the Hercules stream in velocity distributions vary with galactic longitude in nearby stars?
• RQ2 What is the dependence of the Hercules stream's peak velocity on metallicity [Fe/H]?
• RQ3 Why does the peak velocity of the Hercules stream shift with galactic longitude, and what does this imply about its origin?
• RQ4 Are moving groups detected in Hipparcos data more prominent in closer stars (d < 250 pc), and how does their visibility depend on metallicity?
• RQ5 Can the observed kinematic structure be explained by a bar resonant model with a specific angular momentum value?

Key findings

• The Hercules stream is most strongly detected in stars with [Fe/H] > 0.2, indicating a metallicity-dependent kinematic signature.
• The peak velocity of the Hercules stream varies systematically with galactic longitude, suggesting a directional dependence in its origin or dynamics.
• The gap between the stellar stream and more circular orbits is associated with an angular momentum of approximately 1620 km/s kpc, supporting a bar resonant mechanism.
• Moving groups identified in Hipparcos data are most visible in stars within d < 250 pc, highlighting distance-dependent detectability.
• The visibility and kinematic properties of moving groups depend on both galactic longitude and hemisphere, indicating directional asymmetries in local velocity structure.
• Fine-structured features in local velocity distributions vary over spatial scales of a few hundred parsecs, implying non-uniform kinematic substructure in the solar neighborhood.