The tangled warp of the Milky Way
Astronomy & Astrophysics(2024)
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.
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