Researchers at Durham University have presented evidence that the Milky Way experienced a dramatic reorientation following a head-on collision with a dwarf galaxy approximately 10 billion years ago. The incoming galaxy, known as the Gaia Sausage, struck the Milky Way directly, and while the larger galaxy ultimately tore apart and absorbed the interloper’s stars, the impact caused the entire galactic disc to flip by more than 90 degrees into the orientation observed today.
The discovery emerged from investigations into an unexplained phenomenon involving stellar rotation rates in the Milky Way’s halo. While stars in the galactic disc orbit the center at roughly 220 kilometers per second, those in the halo move far more slowly, at approximately 25 kilometers per second. This discrepancy prompted the Durham team to conduct supercomputer simulations of galaxy collisions to identify potential causes.
The simulations demonstrated that galaxies could develop slow-moving halo stars following a direct impact with a dwarf galaxy the size of the Gaia Sausage, accompanied by a subsequent disc flip. According to lead researcher Kirill Batrakov, such a reorientation would require at least several hundred million years to complete. The halo itself is a sparse, roughly spherical cloud of stars surrounding the galaxy, with most stars originating from smaller galaxies that merged with the Milky Way over time.
The Gaia Sausage collision was originally identified in 2018 when astronomers used observations from the European Space Agency’s Gaia mission to map stellar orbits. Researchers discovered that some stars followed unusual, sausage-shaped orbital paths that traced back to a dwarf galaxy fragmenting as it collided with the galactic center. Despite its dwarf classification, the Gaia Sausage contained mass equivalent to more than 10 billion times that of the sun. Astronomers now recognize this merger as the last major collision in the Milky Way’s history and a defining event that created the galactic bulge and dominated the stellar halo.
The research was presented this week at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham. While the next major merger may not occur for several billion years, the Large Magellanic Cloud is eventually expected to merge with the Milky Way, though a smaller merger with the Sagittarius dwarf galaxy is already underway.
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