A Chinese research team led by Dr. HUANG Yang, an associate Professor from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) & University of the Chinese Academy of Sciences (UCAS), pioneered the "Motion picture" method for measuring the precession rate of the Milky Way's disk warp.

 
Using a sample of Cepheid variable stars of different ages, this method allows the precession direction and rate of the Milky Way's warp to be clearly observed. Based on these measurements, the research team revealed that the current dark matter halo of the Milky Way is slightly oblate.

 
The study titled "A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp" has recently been published online in the latest issue of 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 highly 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.

Figure 1: 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. (The artistic image created by HOU Kaiyuan and DONG Zhanxun from the School of Design, Shanghai Jiao Tong University)

 
This study utilized 2,600 young classical Cepheid variable stars discovered by Gaia as tracers, along with precise distance and age data from both Gaia and LAMOST. Using this sample, the researchers applied the "Motion picture" method to construct 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, this study 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 detailed measurements show that the warp's precession rate gradually decreases with radius. Regardless of the origin of the warp, its precession rate and direction are jointly determined by the Galactic inner disk and the dark matter halo. After subtracting the contribution of the Galactic inner disk, the research team 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 measurement provides a crucial anchor point for studying the evolution of the Milky Way's dark matter halo.

Figure 2: The three-dimensional structure of the Galactic disk constructed from Cepheid variable stars of different slices of age, showing the variation of the warp node line with the sample age with the slope represents the warp precession. (Credit: NAOC/UCAS)

 
The study was highly praised by both reviewers: "the motion picture approach to measuring the precession rate is novel and convincing" from Referee #1; "as far as I am aware, this is the first time that the warp is constrained to precess in retrograde direction, and its precession rate is accurately measured" from Referee #2.

 
Dr HUANG Yang is the co-first author and co-corresponding author of the paper. Prof.LIU Jifeng from NAOC, Dr. ZHANG Huawei from Peking University, and Professor SHEN Juntai from Shanghai Jiao Tong University are the co-corresponding authors. Graduate student FENG Qikang from Peking University is the co-first author. This study also includes other astronomers from NAOC, Beijing Normal University, and the University of Notre Dame in the USA.

This paper can be accessed at https://www.nature.com/articles/s41550-024-02309-5

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