Astronomers have discovered that a collision with a dwarf galaxy, named Gaia Sausage, caused the Milky Way to rotate sideways about 10 billion years ago, long before the formation of our solar system. During this event, the Milky Way absorbed the stars of the smaller galaxy while the impact likely flipped its disk more than 90 degrees. Researchers at Durham University used supercomputer simulations to demonstrate this phenomenon, which they term a “disk flip,” suggesting that this process could take several hundred million years.
This research stemmed from investigations into the slow rotation of stars in the Milky Way’s halo—a nearly spherical cloud surrounding the galaxy. Unlike the stars in the Milky Way’s disk, which orbit at approximately 220 km/s, those in the halo move much slower at about 25 km/s. To explain this discrepancy, the team simulated head-on collisions with a galaxy like Gaia Sausage, concluding that such interactions could result in the slower-moving halo stars.
This ancient collision, first identified in 2018, is now regarded as a crucial event in shaping the Milky Way, creating a central bulge and significantly altering its structure. While Gaia Sausage had a mass over 10 billion times that of the Sun, current minor mergers with the Sagittarius dwarf galaxy and the upcoming interaction with the Large Magellanic Cloud are expected to have minimal impact on the Milky Way’s evolution. The findings will be discussed at the Royal Astronomical Society’s conference in Birmingham.
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