The Milky Way’s Dark Matter Halo Has an Oblate Shape: Chinese Scientists

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They constructed a three-dimensional structure of the Milky Way’s disk at different ages over a period of 250 million years

On Tuesday, the Xinhua agency reported that a team of Chinese scientists revealed that the Milky Way’s dark matter halo, the invisible mass that surrounds our galaxy, is slightly oblate.

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The study, conducted by researchers from the University of the Chinese Academy of Sciences (UCAS), in collaboration with several domestic and international research institutions, was published in the latest issue of the journal Nature Astronomy.

Scientists believe that the shape of the dark matter halo is key to understanding the hierarchical formation of the Milky Way. Despite extensive efforts in recent decades, its shape remains a matter of debate, with suggestions ranging from strongly oblate to prolate.

Based on observational data from the European Space Agency’s Gaia satellite and China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), the researchers analyzed about 2,600 Cepheid variable stars of different ages. They applied a new method called “motion picture” to construct a three-dimensional structure of the Milky Way’s disk at different ages over a period of 250 million years.

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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.

By “seeing” how the disk warp evolves with age, the Chinese researchers found that the warp precesses in a retrograde direction at a rate of 0.12 degrees per million years.

“Previously, there was a lack of precise measurement on how the disk warp swings,” said Huang Yang, the co-first author of the study, and an associate professor with UCAS.

Based on their measurements, the research team found that the current dark matter halo enveloping the warp exhibits a slightly oblate ellipsoidal shape.

“Only this shape can explain the precession rate of the warp. This measurement provides a crucial anchor point for studying the evolution of the Milky Way’s dark matter halo and the assembly history of the Galaxy,” Huang said.

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