Milky Way flipped in collision with dwarf galaxy 10bn years ago, astronomers say

Astronomers at Durham University have uncovered evidence that the Milky Way experienced a radical shift 10 billion years ago. A massive head-on collision with a dwarf galaxy, known as the Gaia Sausage, forced our galaxy to flip on its side.

Researchers relied on advanced supercomputer simulations to trace this history. By modeling how galaxies similar to our own react to major impacts, the team found that a direct hit from an object the size of the Gaia Sausage is sufficient to trigger a disc flip. This event lasted for hundreds of millions of years and permanently reoriented the Milky Way into its current structure.

This finding explains a long-standing mystery regarding the behavior of stars in the galactic halo. While disc stars rotate rapidly, stars in the surrounding halo move much slower. The simulations confirm that a violent collision with the Gaia Sausage accounts for this disparity by scattering stars into slower, chaotic orbits during the merger process.

Evidence for this event was first identified in 2018 through data provided by the European Space Agency's Gaia mission. Mapping the paths of stars revealed that many were moving along elongated trajectories, which the research team traced back to the destruction of the incoming dwarf galaxy. This collision is now considered the last major merger in the history of the Milky Way.

The Gaia Sausage contained mass equivalent to 10 billion suns. While other mergers have occurred since, none have matched the scale or impact of this event. Further research into our cosmic history continues to clarify the forces that shaped our current neighborhood.