Milky Way flipped over after head-on collision with Gaia Sausage

Our Milky Way galaxy may have flipped onto its side after a violent collision with a rogue galaxy billions of years before our solar system even formed, astronomers have discovered. The cataclysmic event, estimated to have occurred about 10 billion years ago, involved the Milky Way smashing head-on into a dwarf galaxy dubbed Gaia Sausage.

While the Milky Way gradually absorbed the smaller galaxy's stars, the ensuing chaos likely caused its disk to invert by more than 90 degrees, leading to its current shape, researchers suggest. Supercomputer simulations conducted by astronomers at Durham University indicate that galaxies similar to ours, when colliding with Gaia Sausage, are prone to a "disk inversion" phenomenon that dramatically reorients them over hundreds of millions of years.

This conclusion stems from investigations into the surprisingly slow rotation of stars within the Milky Way's halo, a sparse, spherical region surrounding the galaxy. Most of these halo stars originated in smaller galaxies that merged with the Milky Way over time. Observations from the European Space Agency's Gaia mission show that stars in the Milky Way's disk orbit its center at about 220 kilometers per second, while halo stars move much slower, around 25 kilometers per second.

The Durham researchers' simulations revealed that galaxies acquire slowly rotating halo stars after head-on collisions with neighboring dwarf galaxies like Gaia Sausage, followed by disk inversion. This ancient collision, first identified in 2018 through Gaia mission data, is now considered the last major merger in the Milky Way's history, fundamentally reshaping it and creating its central bulge and halo structure.