Scientists reveal the shape of the Milky Way’s dark matter halo with a “motion-picture” method

In this artist's impression, the galactic disk warp "dances gracefully" under the torque of the dark matter halo. Credit: Hou Kaiyuan and Dong Zhanxun from the School of Design, Shanghai Jiao Tong University.

Scientists have developed a groundbreaking “motion-picture” method to study the shape of the Milky Way’s dark matter halo.

An international team used this method to measure the movement of the Milky Way’s disk warp, offering new insights into the galaxy’s structure.

This research, published in Nature Astronomy, shows that the dark matter halo of our galaxy is slightly oblate, meaning it is somewhat flattened.

In the universe, about one-third of disk galaxies, including our Milky Way, are not perfect flat disks but have a warped shape similar to a bent potato chip.

This phenomenon is called a disk warp.

Our galaxy’s disk, much like a spinning top, undergoes a motion called precession due to the pull of the surrounding dark matter halo.

Measuring the precession rate and direction of this warp has been a challenge for astronomers.

Previous attempts to measure the warp relied on indirect methods, which were often inaccurate due to various disturbances affecting the tracers used.

However, this study used a new approach involving 2,600 Cepheid variable stars.

These stars, discovered by the Gaia spacecraft, have precise distance and age data from both Gaia and the LAMOST survey. By observing these stars, researchers could create a detailed three-dimensional map of the Milky Way’s disk across different star ages.

By analyzing how the warp changes over time, the scientists found that the Milky Way’s warp moves in a retrograde direction, meaning it moves backward compared to the galaxy’s rotation.

The precession rate is 2 km/s per kiloparsec, or 0.12 degrees per million years. Detailed measurements also revealed that this rate decreases with distance from the galactic center.

The research showed that the Milky Way’s dark matter halo is slightly oblate, with a flattening value (q) between 0.84 and 0.96. This means the halo is not perfectly spherical but somewhat squished.

These findings provide important information about the dark matter halo’s structure, which plays a crucial role in the galaxy’s evolution.

This study offers a key reference point for understanding the Milky Way’s dark matter halo and its effects on the galaxy.

By using the “motion-picture” method with Cepheid variable stars, scientists have gained a clearer picture of how our galaxy’s disk warp moves and changes over time. This research helps us better understand the complex dynamics of the Milky Way and the mysterious dark matter that surrounds it.