The tangled warp of the Milky Way

TL;DR

This study analyzes the Milky Way's warp using Gaia DR3 data, finding it precesses rapidly and is dynamically evolving, challenging existing models of warp formation.

Contribution

It introduces a kinematic model of a precessing warp fitted to Gaia data, revealing the warp's rapid precession and dynamic evolution.

Findings

Warp's line of nodes offset by 40 degrees from Sun

Warp precession speed around 13 km/s/kpc

Including radial velocities worsens model fit

Abstract

We determine the influence of the Milky Way's warp on the kinematics of stars across the disc, and therefore measure its precession rate and line of nodes under different assumptions. We do this by applying Jeans' first equation to a model of a rigidly precessing warp. The predictions of these models are fit to the average vertical velocities of stars with measured line-of-sight velocities in Gaia DR3 data. We test models in which the warp's line of nodes and precession speed are fixed, and models in which they are allowed to vary linearly with radius. We also test models in which the velocity of stars radially in the disc is included in Jeans' equation. The kinematic data is best fit by models with a line of nodes that is 40 degrees offset from the Sun's Galactic azimuth, significantly leading the line of nodes found from the positions of stars. These models have a warp precession speed of around 13 km/s/kpc in the direction of Galactic rotation, close to other recent estimates. We find that including the velocity of stars radially in the disc in our kinematic model leads to a significantly worse fit to the data, and implausible warp parameters. We conclude that the Milky Way's warp appears to be rapidly precessing, but the structure and kinematics of the warped disc are not consistent within the approximation of a fixed, precessing, warp shape. This implies that the Milky Way's warp is dynamically evolving, which is a challenge to models of the warp's creation, and must be considered in the context of other known disturbances of the disc.