In the nearby universe, about one-third of disk galaxies are not perfect disks but exhibit a warped shape similar to a potato chip. This phenomenon, known as a disk warp, is also present in the Milky Way. It is widely believed that the warp originates from the rotational plane of the outer disk stars deviating from the symmetry plane of the surrounding dark matter halo. The tilted, rotating Galactic disk, much like a spinning top, experiences precession due to the torque exerted by the surrounding dark matter halo.
The accurate measurement of this dynamic parameter, including both direction and rate, has been a topic of much debate. Previous measurements relied on indirect kinematic methods, where the tracers were influenced by dynamical perturbations or heating effects, limiting their accuracy and precision.
The study utilized 2,600 young classical Cepheid variable stars discovered by Gaia as tracers, combined with precise distance and age data from both Gaia and LAMOST. Using the "motion picture" method, the researchers constructed the three-dimensional structure of the Milky Way's disk across populations of different ages, all younger than 250 Myr.
By analyzing how the disk warp evolves with age, the researchers discovered that the warp precesses in a retrograde direction at a rate of 2 km/s/kpc (or 0.12 degrees per million years).
Further measurements showed that the warp's precession rate gradually decreases with radius. The precession rate and direction were determined jointly by the Galactic inner disk and the dark matter halo.
After subtracting the contribution of the Galactic inner disk, the researchers concluded that the current dark matter halo enveloping the warp has a slightly oblate ellipsoidal shape with a flattening ratio q between 0.84 and 0.96 for the equipotential surfaces. This shape explains the remaining precession rate of the warp.
This study provides a critical anchor point for understanding the evolution of the Milky Way's dark matter halo.
Research Report:A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp
