Astronomers have found evidence suggesting the Milky Way was flipped by more than 90 degrees after a head-on collision with a dwarf galaxy around 10 billion years ago.
Researchers at Durham University said the collision involved a dwarf galaxy known as the Gaia Sausage, which was torn apart and absorbed by the Milky Way. They believe the impact gradually reoriented the Milky Way's disc into its present position over several hundred million years.
The findings are based on supercomputer simulations of Milky Way-like galaxies and are being presented at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.
The research was prompted by a long-standing mystery surrounding the slow rotation of stars in the Milky Way's halo, the sparse cloud of stars surrounding the galaxy. While stars in the Milky Way's disc orbit the galactic centre at about 220 km per second, halo stars move much more slowly, at around 25 km per second.
Lead researcher Kirill Batrakov said the simulations showed that galaxies struck head-on by a dwarf galaxy the size of the Gaia Sausage developed slow-moving stellar haloes after undergoing a "disc flip".
The Gaia Sausage collision was first identified in 2018 using observations from the European Space Agency's Gaia mission. Astronomers found stars travelling in distinctive elongated orbits, indicating they originated from a dwarf galaxy that had plunged into the centre of the Milky Way.
Scientists now regard the event as the Milky Way's last major merger, believing it reshaped the galaxy, formed its central bulge and contributed much of its stellar halo.
Although classified as a dwarf galaxy, the Gaia Sausage contained stars, gas and dark matter with a combined mass exceeding 10 billion times that of the Sun.
Researchers said the Milky Way is expected to undergo another major merger in a few billion years when the Large Magellanic Cloud merges with it. A separate merger with the smaller Sagittarius dwarf galaxy is already underway but is expected to have a much smaller impact.