Chinese researchers have shed light on a surprising phenomenon in our galaxy: the spiral disk of the Milky Way is undergoing retrograde precession, influenced by an immense mass of dark matter forming an invisible halo. This intriguing behavior opens up new perspectives on galactic dynamics.
Artistic interpretation of the deformed disk of the Milky Way, surrounded by a slightly flattened halo of dark matter. Credit: Kaiyuan Hou and Zhanxun Dong (School of Design, Shanghai Jiao Tong University)
Nearly a third of spiral galaxies exhibit a distinct warp in their disk structure. This deformation, often due to past galactic collisions, can also result from interactions with satellite galaxies, the intergalactic magnetic field, and the influx of vast gas clouds. However, for the Milky Way, the primary factor maintaining this deformation is the dark matter halo.
This phenomenon of deformation is not static. It undergoes precession, which means a change in alignment relative to the galaxy's rotation axis, similar to the wobbling motion of a spinning top. Accurately measuring this precession has long posed challenges, as previous methods relied on giant stars whose vertical movements proved imprecise.
Astronomers led by Yang Huang of the Chinese Academy of Sciences used Cepheid variable stars as new tracers to measure this precession with unprecedented precision. Cepheids, massive pulsating stars, provide a precise measure of their intrinsic luminosity, allowing for a detailed mapping of the deformation.
Using data from the European Space Agency's Gaia satellite, Huang's team identified a sample of 2,613 Cepheids of various ages. By grouping these stars by age brackets and mapping them, they observed the evolution of the galactic disk's deformation over the past 200 million years, discovering a retrograde precession.
The discovered precession rate is 0.12 degrees per million years, decreasing with distance from the galactic center, which could lead to increased disk deformation in the long term. This discovery provides clues about the flattened shape of the dark matter halo, enriching theoretical models of the Milky Way's formation and evolution.
These findings, published on June 27 in
Nature Astronomy, help refine our understanding of dark matter and the cosmic history of our galaxy.