New research indicates the Milky Way experienced a major shift in disk orientation billions of years ago. This finding may resolve a puzzle about the galaxy’s structure. Astronomers at Durham University used supercomputer models of similar galaxies to show that those with slowly rotating stellar halos often underwent a disk flip exceeding 90 degrees.
The study was shared at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham.
Most Milky Way stars reside in its flat spiral disk, encircled by a sparse halo of stars from past mergers with smaller galaxies. Data from the Gaia spacecraft revealed the halo rotates slowly, a fact previously unexplained.
Researchers examined 25 Milky Way-like systems in the Auriga simulations across billions of years. Galaxies with the slowest halos had undergone both a direct major merger and a disk reorientation.
The Milky Way is known to have merged head-on with the Gaia-Sausage-Enceladus dwarf galaxy around 10 to 11 billion years ago. This event likely triggered the disk flip, according to lead researcher Kirill Batrakov.
Such a flip implies many stars, possibly including the Sun, once followed different paths. It may also offer indirect insights into the galaxy’s dark matter halo.
The Milky Way serves as a key laboratory for galaxy evolution studies. A confirmed past flip would add an important element to models of how similar systems formed, based solely on current observations.
