Milky Way flipped in collision with dwarf galaxy 10bn years ago, astronomers say
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- Durham University astronomers used supercomputer simulations to show that a head-on collision with a dwarf galaxy called the Gaia Sausage roughly 10 billion years ago flipped the Milky Way's disc by more than 90 degrees into its current orientation.
- Lead researcher Kirill Batrakov said the disc flip probably took at least a few hundred million years to complete, and the findings were presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.
- The collision, now recognized as the Milky Way's last major merger, fundamentally reshaped the galaxy by creating the central bulge and populating the stellar halo, where stars still rotate at about 25 km/s compared with about 220 km/s for disc stars.
- The Gaia Sausage was first identified in 2018 via European Space Agency Gaia mission data, which revealed stars on radical, sausage-shaped orbits traced back to a dwarf galaxy breaking apart as it ploughed into the Milky Way's heart.
- Though classed as a dwarf galaxy, the Gaia Sausage carried stars, gas and dark matter amounting to more than 10 billion times the mass of the sun before being torn apart and absorbed.
- Batrakov noted a smaller Sagittarius dwarf galaxy merger is already under way but will have a much less dramatic impact, with the next major upheaval not expected for a few billion years when the Large Magellanic Cloud merges with the Milky Way.
Why it matters: The finding reframes the Milky Way's defining historical event: the Gaia Sausage collision wasn't just a merger but a structural reorientation that explains why halo stars orbit nearly ten times slower than disc stars. It also sets the timeline for the galaxy's next chapter, with the Large Magellanic Cloud merger still billions of years away.
