A team of researchers led by Dr. Huang Yang from the National Astronomical Observatories of the Chinese Academy of Sciences has made a groundbreaking discovery about the shape of the Milky Way’s dark matter halo. By using a novel “motion picture” method, they measured the precession rate of the galaxy’s disk warp with unprecedented accuracy.
The team analyzed 2,600 young classical Cepheid variable stars discovered by Gaia and combined them with precise distance and age data from both Gaia and LAMOST. Their findings revealed that the dark matter halo is slightly oblate, meaning it is shaped like a flattened ellipsoid. This discovery provides a crucial anchor point for studying the evolution of the Milky Way’s dark matter halo. The research was published online in Nature Astronomy and marks a significant milestone in our understanding of galaxy formation and evolution.
Unveiling the Shape of the Milky Way’s Dark Matter Halo
The Milky Way, a typical disk galaxy, exhibits a warped shape resembling a potato chip, a phenomenon observed in nearly one-third of nearby disk galaxies. This warp feature is believed to originate from the rotational plane of outer disk stars deviating from the symmetry plane of the surrounding dark matter halo. As a result, the tilted, rotating Galactic disk undergoes precession due to the torque exerted by the surrounding dark matter halo.
Measuring this important dynamic parameter, both in direction and rate, has been widely debated among astronomers. Previous measurements relied on indirect kinematic methods, where tracers used are subject to dynamical perturbations or heating effects, greatly limiting their accuracy and precision. However, a recent study led by Dr. HUANG Yang from the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC) has pioneered a novel approach to measuring the precession rate of the Milky Way’s disk warp.
The “Motion Picture” Method: A Breakthrough in Measuring Precession Rate
The research team employed a sample of 2,600 young classical Cepheid variable stars discovered by Gaia as tracers, along with precise distance and age data from both Gaia and LAMOST. By constructing the three-dimensional structure of the Milky Way’s disk across populations of various ages but all younger than 250 Myr, the researchers were able to “see” how the disk warp evolves with age.
This innovative approach, dubbed the “motion picture” method, allowed for a clear observation of the precession direction and rate of the Milky Way’s warp. The results showed that the warp precesses in a retrograde direction at a rate of 2 km/s/kpc (or 0.12 degrees per million years). Furthermore, measurements revealed that the warp’s precession rate gradually decreases with radius.
Unraveling the Dark Matter Halo’s Shape
By jointly determining the Galactic inner disk and dark matter halo’s contributions to the warp’s precession rate and direction, the researchers found that the current dark matter halo enveloping the warp exhibits a slightly oblate ellipsoidal shape. This shape is characterized by a flattening ratio q between 0.84 and 0.96 for the equipotential surfaces.
The study provides a crucial anchor point for understanding the evolution of the Milky Way’s dark matter halo. The revealed shape of the dark matter halo can only explain the remaining precession rate of the warp, shedding light on the intricate dynamics at play in our galaxy.
Implications and Future Directions
This groundbreaking research has significant implications for our understanding of galaxy evolution and the role of dark matter halos. The “motion picture” method offers a powerful tool for studying the dynamics of disk galaxies, enabling astronomers to probe the properties of dark matter halos with unprecedented precision.
Future studies can build upon this foundation, exploring the connections between dark matter halo shapes, galaxy morphology, and the evolution of disk galaxies. As our understanding of these complex processes deepens, we may uncover new insights into the mysteries of the universe, ultimately refining our models of galaxy formation and evolution.
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