Milky Way’s Disc Flipped by Ancient Gaia Sausage Collision

Astronomers have presented new evidence that a head‑on collision with a dwarf galaxy, dubbed the Gaia Sausage, turned the Milky Way’s disc more than 90 degrees about 10 billion years ago, well before the Solar System formed. The impact shredded the dwarf galaxy, incorporated its stars into the Milky Way and generated a chaotic disturbance that slowly reoriented the galactic disc over several hundred million years.

The discovery explains the stark speed difference between stars in the Milky Way’s disc (≈220 km s⁻¹) and those in its extended halo (≈25 km s⁻¹). Researchers at Durham University used super‑computer simulations to show that galaxies experiencing such direct mergers develop slow‑moving stellar haloes after a “disc flip”. The findings were presented at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham and build on the 2018 identification of the Gaia Sausage using data from the ESA Gaia mission.

The event is now regarded as the Milky Way’s last major merger, shaping the galaxy’s central bulge, halo and overall orientation, with future mergers (e.g., with the Large Magellanic Cloud) expected to further modify it.