Researchers at Durham University have presented findings suggesting that the Milky Way experienced a dramatic reorientation following a head-on collision with a dwarf galaxy approximately 10 billion years ago. The wayward galaxy, known as the Gaia Sausage, struck the Milky Way directly, and while the larger galaxy tore the interloper apart and absorbed its stellar material, the impact set off a chain of events that rotated the Milky Way’s disc by more than 90 degrees into its current orientation.
Using supercomputer simulations, the Durham team modeled how Milky Way-like galaxies would respond to direct impacts from objects the size of the Gaia Sausage. The results indicated that such collisions would produce a “disc flip” effect, gradually reorienting the entire galactic structure over several hundred million years or more. This mechanism emerged as a leading explanation for a longstanding astronomical puzzle regarding the Milky Way’s stellar halo.
Observations from the European Space Agency’s Gaia mission had revealed a significant discrepancy in rotation speeds within the galaxy. Stars in the Milky Way’s disc orbit the galactic center at approximately 220 kilometers per second, while stars in the stellar halo—the roughly spherical cloud of older stars surrounding the galaxy—move far more slowly at about 25 kilometers per second. The Durham simulations demonstrated that this velocity difference was consistent with a major collision followed by a disc flip.
The evidence for the Gaia Sausage collision first emerged in 2018 when astronomers used Gaia mission data to trace the orbital paths of stars through the Milky Way. Some stars displayed unusually elongated, sausage-shaped trajectories that, when traced backward, pointed to a dwarf galaxy disrupting as it collided with the galactic center. The Gaia Sausage, despite its dwarf classification, contained stellar material, gas, and dark matter totaling more than 10 billion solar masses. The research team presented these findings this week at the Royal Astronomical Society’s National Astronomy Meeting.
Astronomers now view the ancient collision as the Milky Way’s most recent major merger and a transformative moment in galactic history, responsible for creating the central bulge and shaping the stellar halo. A future merger with the nearby Large Magellanic Cloud may occur in a few billion years, while an ongoing merger with the smaller Sagittarius dwarf galaxy is expected to produce considerably less dramatic effects given the current mass difference and the Milky Way’s greater size.
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