A research team led by Dr. HUANG Yang from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) pioneered the “motion picture” method for measuring the precession rate of the Milky Way’s disk warp. This method allows the clear observation of the precession direction and rate of the Milky Way’s warp using a sample of Cepheid variable stars of different ages. Based on these measurements, the team revealed that the current dark matter halo of the Milky Way is slightly oblate. The study was published online in Nature Astronomy.  

In the nearby universe, nearly one-third of disk galaxies are not perfect disks but exhibit a warped shape resembling a potato chip. Astronomers refer to this phenomenon as a disk warp. The Milky Way, as a typical disk galaxy, also has this warp feature. It is generally 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. This tilted, rotating Galactic disk, much like a spinning top, inevitably undergoes precession due to the torque exerted by the surrounding dark matter halo.  

However, the measurement of this important dynamic parameter, both in direction and rate, has been widely debated. This is because previous measurements relied on indirect kinematic methods, where the tracers used are subject to dynamical perturbations or heating effects, greatly limiting their accuracy and precision.  

This study used 2,600 young classical Cepheid variable stars discovered by Gaia as tracers, along with precise distance and age data from both Gaia and LAMOST. The researchers, using the “motion picture” method, constructed the three-dimensional structure of the Milky Way’s disk across populations of various ages but all younger than 250 Myr.  

By “seeing” how the disk warp evolves with age, the researchers found 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. Regardless of the origin of the warp, its precession rate and direction were jointly determined by the Galactic inner disk and the dark matter halo.   

After subtracting the contribution of the Galactic inner disk, the researchers found that the current dark matter halo enveloping the warp exhibits a slightly oblate ellipsoidal shape with a flattening ratio q between 0.84 and 0.96 for the equipotential surfaces. Currently, only this shape can explain the remaining precession rate of the warp.   

This study provides a crucial anchor point for studying the evolution of the Milky Way’s dark matter halo.  

Right panel: the spinning top precesses under the torque from gravity; Left panel: similar to the spinning top, the Galactic disk warp "dances gracefully" under the torque of the dark matter halo. (Image by HOU Kaiyuan and DONG Zhanxun from the Shanghai Jiao Tong University)