Evidence of a dynamically evolving Galactic warp

TL;DR

This paper measures the precession rate of the Milky Way's warp using Gaia data, providing insights into its dynamical evolution and suggesting recent satellite interactions as a cause.

Contribution

It introduces a method to quantify the warp's precession rate using stellar kinematics, revealing the warp's dynamic nature and potential recent external influences.

Findings

The warp precesses at approximately 10.86 km/s/kpc.

The precession is about one third of the Sun's orbital velocity.

Results support a recent satellite encounter as the warp's cause.

Abstract

In a cosmological setting, the disc of a galaxy is expected to continuously experience gravitational torques and perturbations from a variety of sources, which can cause the disc to wobble, flare and warp. Specifically, the study of galactic warps and their dynamical nature can potentially reveal key information on the formation history of galaxies and the mass distribution of their halos. Our Milky Way presents a unique case study for galactic warps, thanks to the detailed knowledge of its stellar distribution and kinematics. Using a simple model of how the warp's orientation is changing with time, we here measure the precession rate of the Milky Way's warp using 12 million giant stars from Gaia Data Release 2, finding that it is precessing at $10.86 \pm 0.03_{stat} \pm 3.20_{sys}$ km/s/kpc in the direction of Galactic rotation, about one third the angular velocity at the Sun's position in the Galaxy. The direction and magnitude of the warp's precession rate favour the scenario that the warp is the results of a recent or ongoing encounter with a satellite galaxy, rather than the relic of the ancient assembly history of the Galaxy.