A catastrophic collision that flipped the Milky Way onto its side around 10 billion years ago has been confirmed by astronomers. The devastating head-on impact, which occurred long before our solar system formed, was with a dwarf galaxy known as the Gaia Sausage. Researchers at Durham University have used supercomputer simulations to recreate the aftermath of this ancient collision, revealing that it would have taken hundreds of millions of years for the galaxy's disc to reorient by more than 90 degrees.
The team's findings support the theory that a direct hit from a dwarf galaxy the size of the Gaia Sausage could explain the slow rotation of stars within the Milky Way's stellar halo. While main-disc stars orbit at approximately 220 kilometres per second, those in the sparse, ball-shaped halo surrounding the galaxy move much slower – around 25 kilometres per second.
The discovery was made possible by investigating a longstanding astronomical enigma: why the slow-moving halo stars don't seem to fit the expected rotation patterns. The Durham team's simulations suggest that galaxies which experience head-on collisions and subsequent disc flips would indeed exhibit these slow-moving halo stars, lending weight to their theory.
The Gaia Sausage galaxy itself was first identified in 2018 by an international team of astronomers using data from the European Space Agency's Gaia mission. They mapped the paths of stars through the Milky Way and pinpointed a distinct population with radical, sausage-shaped orbits – a remnant of the dwarf galaxy that disintegrated as it plunged directly into the heart of our own galaxy.
Despite being classified as a dwarf galaxy, the Gaia Sausage had an impressive mass, exceeding 10 billion times that of the Sun. Its impact on the Milky Way was profound, fundamentally reshaping our galaxy and forming the central bulge and dominating the stellar halo. Another smaller merger with the Sagittarius dwarf is currently underway, but its effects are expected to be significantly less dramatic.