Researchers at Durham University have identified evidence suggesting the Milky Way underwent a dramatic reorientation following a direct collision with a dwarf galaxy approximately 10 billion years ago, long before the formation of Earth’s solar system. Using advanced supercomputer simulations, the team demonstrated that such head-on impacts between Milky Way-like galaxies and objects similar in scale to the Gaia Sausage could trigger a “disc flip” effect, rotating the galaxy’s disc by more than 90 degrees into its current orientation. The transformation would have unfolded gradually over several hundred million years, the researchers noted.
The discovery emerged from investigations into an unusual astronomical puzzle: stars in different regions of the Milky Way rotate at markedly different speeds. Data from the European Space Agency’s Gaia mission reveals that stars in the galactic disc orbit the galaxy’s center at approximately 220 kilometers per second, while those in the surrounding stellar halo move far more slowly at about 25 kilometers per second. The Durham team’s simulations provided a potential explanation for this discrepancy, showing that slow-moving halo stars could result from the type of collision that also caused the disc flip.
The dwarf galaxy, designated the Gaia Sausage, was identified in 2018 when astronomers mapped unusual star trajectories through the Milky Way using Gaia mission data. Some stars were found following distinctive sausage-shaped orbital paths, which traced back to a dwarf galaxy breaking apart during impact with the galactic center. Though classified as a dwarf galaxy, the colliding object contained stars, gas, and dark matter totaling more than 10 billion times the sun’s mass. Astronomers now regard this merger as the Milky Way’s last major collision and a transformative event that created the galaxy’s central bulge and shaped its stellar halo.
The research is being presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham. While the Milky Way faces eventual merger with the nearby Large Magellanic Cloud, that event may not occur for several billion years. A separate, ongoing merger with the smaller Sagittarius dwarf galaxy is expected to have minimal impact on the Milky Way given the relative sizes and masses involved, according to researchers.
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