Astronomers have uncovered evidence indicating that the Milky Way galaxy experienced a dramatic reorientation approximately 10 billion years ago following a head-on collision with a dwarf galaxy known as the Gaia Sausage. This event, which occurred long before the formation of the solar system, is believed to have flipped the Milky Way’s disc by more than 90 degrees to its current position.

Researchers at Durham University utilized supercomputer simulations to investigate the effects of direct collisions between Milky Way-like galaxies and dwarf galaxies similar to the Gaia Sausage. The simulations suggest that such a head-on encounter can cause a “disc flip,” a process taking several hundred million years, which significantly reorients the galactic disc.

The study was motivated by a longstanding question regarding the distinct rotational speeds observed in different parts of the Milky Way. While stars in the galaxy’s disc orbit the galactic center at approximately 220 kilometers per second, stars populating the more diffuse stellar halo rotate much more slowly, at around 25 kilometers per second. The halo, a roughly spherical cloud of stars surrounding the galaxy, primarily consists of stars captured from smaller galaxies that merged with the Milky Way over time.

The Gaia Sausage collision is recognized as the galaxy’s last major merger, an event that played a fundamental role in shaping the Milky Way’s structure. It contributed to the formation of the central bulge and dominated the stellar halo’s composition. Despite being classified as a dwarf galaxy, the Gaia Sausage contained stars, gas, and dark matter totaling more than 10 billion solar masses.

Looking ahead, astronomers anticipate another significant interaction in several billion years with the Large Magellanic Cloud, a dwarf galaxy currently orbiting the Milky Way. This future merger is expected to cause notable, though less dramatic, changes to the galaxy’s structure than those resulting from the Gaia Sausage collision. Meanwhile, the Milky Way is currently undergoing a smaller merger with the Sagittarius dwarf galaxy, but this event is projected to have minimal impact due to its relatively low mass compared to past encounters and the galaxy’s present size.

Kirill Batrakov, the project’s lead researcher, noted that the influence of smaller galaxies like Sagittarius is limited in comparison to the Gaia Sausage, emphasizing that the Milky Way’s increased mass over time reduces the effects of recent and ongoing mergers.