Milky Way Flipped 90 Degrees After 10-Billion-Year-Old Gaia Sausage Collision

Q: What role did invisible dark matter play in the cosmic collision that flipped our galaxy on its side?

Dark matter was likely the main gravitational engine of the collision that reoriented the Milky Way. The Gaia Sausage dwarf galaxy was not just a clump of stars and gas; like all galaxies, it was embedded in a much larger halo of invisible dark matter, and that halo would have carried much of the merger’s mass. Because dark matter interacts primarily through gravity, not through light or gas-like drag, it would have helped drive the dwarf galaxy straight through the Milky Way’s own dark matter halo. In a head-on merger, the two halos would have strongly disturbed the Milky Way’s overall gravitational field, transferring energy and angular momentum and gradually torquing the stellar disc until it flipped by more than 90 degrees over hundreds of millions of years. That same dark-matter-dominated impact also helps explain the Milky Way’s odd stellar halo today. Durham University’s simulations show that a radial collision of this kind can leave halo stars rotating very slowly, matching observations from Gaia: about 25 km/s in the halo versus roughly 220 km/s in the main disc. So dark matter did not “hit” the Milky Way in the ordinary sense. Its role was to supply most of the hidden mass and gravitational pull that made the merger violent enough to reshape the galaxy, build much of the halo and bulge, and ultimately tip the Milky Way onto a new orientation.

3 articles · Updated · The Guardian · Jul 21

Durham University astronomers found evidence that a head-on collision with the Gaia Sausage about 10 billion years ago flipped the Milky Way’s disc by more than 90 degrees.
Supercomputer simulations linked that ancient impact to another puzzle: the Milky Way’s halo stars rotate at only about 25 km a second, versus roughly 220 km a second for stars in the disc.
The team said the reorientation was slow rather than sudden, likely unfolding over at least a few hundred million years after the dwarf galaxy was torn apart and absorbed.
Gaia data first exposed the merger in 2018 through stars on extreme sausage-shaped orbits; astronomers now view it as the Milky Way’s last major merger and a defining event in its history.
Researchers are presenting the findings this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham, while noting later mergers such as Sagittarius should be far less disruptive.