Tracking Galactic Motions: Shapes and Tilts of Stellar Velocity Patterns

The paper by Dmytrenko et al. explores the spatial orientation and shape of velocity ellipsoids for red giants and subgiants using data from Gaia’s third data release (Gaia DR3). The study investigates the three-dimensional motion of stars across the Galactic plane, revealing new insights into the Milky Way's kinematics and the impact of gravitational distortions.

Introduction

The authors leverage Gaia DR3, which provides precise astrometric data including stellar parallaxes and velocities, to study stellar motions far beyond the solar neighborhood. They focus on "velocity ellipsoids," geometric representations of residual stellar velocities, and analyze their orientation and shape. These ellipsoids offer clues about the Galaxy’s dynamics and potential distortions such as warps or other non-axisymmetric structures.

Coordinate Systems

To analyze stellar motions, the study employs a local Galactic Cartesian system centered at various points in the Galactic mid-plane. By focusing on red giants and subgiants within 1-kiloparsec (kpc) spheres, the team constructs centroids and velocity ellipsoids for these stellar groups. The sample includes over 14 million stars, carefully filtered to ensure accuracy.

Method

The orientation and shape of velocity ellipsoids are derived using mathematical principles, such as eigenvalues and eigenvectors, to characterize the semi-axes of the ellipsoids. These axes reflect anisotropic stellar motions. The study accounts for uncertainties in the data, including those in stellar parallaxes and velocities, which could influence the precision of the ellipsoid measurements.

Results

The analysis reveals that the largest axis of the velocity ellipsoid often deviates significantly from the expected direction, particularly in regions of the Galactic plane beyond 10 kpc. These deviations are linked to kinematic distortions like the Galactic warp. Additionally, the authors find that the tilt of the ellipsoids increases with distance, suggesting a more complex velocity field as one moves away from the Galactic center.

The study also examines the shape of velocity ellipsoids, which varies with Galactocentric distance. Near the center, ellipsoids exhibit significant elongation, reflecting the impact of gravitational disturbances. Beyond 6 kpc, the motions become more ordered, with smaller velocity dispersions.

Comparison

The findings are compared with earlier studies that analyzed deformation velocity tensors. While both approaches identify key kinematic features, the use of three-dimensional spatial velocities in this work provides more detailed insights, particularly into deviations caused by non-axisymmetric structures.

Conclusions

Dmytrenko et al. conclude that velocity ellipsoids are a powerful tool for understanding Galactic dynamics. Their findings confirm that the Milky Way's kinematics deviate from a simple axisymmetric model, especially at greater distances. These results offer valuable kinematic signatures of the Galactic warp and provide a foundation for future research into the Galaxy's structure.

Source: Dmytrenko