A cataclysmic head-on collision with a wayward dwarf galaxy roughly 10 billion years ago likely flipped the Milky Way onto its side. Astronomers at Durham University, using advanced supercomputer simulations, have determined that this event—involving a galaxy known as the Gaia Sausage—reoriented the Milky Way’s disc by more than 90 degrees into its current position. According to lead researcher Kirill Batrakov, the dramatic, slow-moving process of this disc flip likely spanned at least a few hundred million years.
The discovery emerged as researchers sought to explain the unusually slow rotation of stars within the Milky Way’s stellar halo. While stars in the galactic disc rotate around the center at approximately 220km per second, those in the halo move at a significantly slower pace of about 25km per second. By simulating the evolution of galaxies similar to our own, the team found that a head-on impact with a body the size of the Gaia Sausage naturally results in the creation of a slow-moving stellar halo and a substantial disc flip.
The Gaia Sausage, which contained stars, gas, and dark matter totaling more than 10 billion times the mass of the sun, was torn apart by the Milky Way during the merger. This event, which researchers consider the galaxy's last major merger, fundamentally reshaped the Milky Way, helping to form the central bulge and dominating the composition of the stellar halo. The collision was first identified in 2018 through European Space Agency Gaia mission observations, which revealed stars tracing radical, sausage-shaped orbits that served as evidence of the dwarf galaxy’s destruction.
These findings are being presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham. While the Milky Way continues to interact with smaller neighbors, such as the Sagittarius dwarf galaxy, astronomers do not expect a repeat of the Gaia Sausage’s impact. The Sagittarius dwarf is significantly less massive, and the Milky Way is now much larger than it was 10 billion years ago. Looking ahead, the next major galactic upheaval is projected to occur in a few billion years when the Large Magellanic Cloud merges with the Milky Way.
