Exploring the Galactic Warp Through Asymmetries in the Kinematics of the Galactic Disk

TL;DR

This study uses Gaia DR2, StarHorse, and APOGEE data to analyze the Milky Way's warp through stellar kinematics, revealing asymmetries, ripples, and a lopsided warp structure, and modeling its properties.

Contribution

It provides the most detailed, radially expansive analysis of Galactic warp kinematics, incorporating new velocity trends and a simple analytical warp model.

Findings

Vertical velocity decreases at R=13 kpc and Lz=2800 kpc km/s.

Detected ripples in vertical and radial velocities superposed on main trends.

Warp appears lopsided with asymmetries in azimuthal velocity trends.

Abstract

Previous analyses of large databases of Milky Way stars have revealed the stellar disk of our Galaxy to be warped and that this imparts a strong signature on the kinematics of stars beyond the solar neighborhood. However, due to the limitation of accurate distance estimates, many attempts to explore the extent of these Galactic features have generally been restricted to a volume near the Sun. By combining Gaia DR2 astrometric solution, StarHorse distance and stellar abundances from the APOGEE survey, we present the most detailed and radially expansive study yet of the vertical and radial motions of stars in the Galactic disk. We map stellar velocity with respect to their Galactocentric radius, angular momentum, and azimuthal angle and assess their relation to the warp. A decrease in vertical velocity is discovered at Galactocentric radius $R=13\ \text{kpc}$ and angular momentum $L_z=2800\ \text{kpc}\ \text{km}\ \text{s}^{-1}$. Smaller ripples in vertical and radial velocity are also discovered superposed on the main trend. We also discovered that trends in the vertical velocity with azimuthal angle are not symmetric about the peak, suggesting the warp to be lopsided. To explain the global trend in vertical velocity, we built a simple analytical model of the Galactic warp. Our best fit yields a starting radius of $8.87^{+0.08}_{-0.09}\ \text{kpc}$ and precession rate of $13.57^{+0.20}_{-0.18}\ \text{km}\ \text{s}^{-1}\ \text{kpc}^{-1}$. These parameters remain consistent across stellar age groups, a result that supports the notion that the warp is the result of an external, gravitationally induced phenomenon.