Researchers at Durham University have produced evidence suggesting the Milky Way experienced a dramatic reorientation following a head-on collision with a dwarf galaxy roughly 10 billion years ago. Using supercomputer simulations, the team demonstrated that such an impact would have caused the galaxy’s disc to flip by more than 90 degrees, aligning with the orientation observed today. The process of reorientation would have unfolded gradually over hundreds of millions of years, according to lead researcher Kirill Batrakov.
The investigation began with an examination of an unusual characteristic in the Milky Way’s structure: stars in its halo rotate at significantly slower speeds than those in the main disc. Data from the European Space Agency’s Gaia mission shows that disc stars orbit the galactic center at approximately 220 kilometers per second, while halo stars move at only about 25 kilometers per second. This discrepancy prompted researchers to explore potential explanations through computational modeling.
The colliding dwarf galaxy, known as the Gaia Sausage, was identified through earlier observations of unusual star patterns within the Milky Way. Astronomers traced certain stars following distinctive sausage-shaped orbits back to their origin in a disrupting dwarf galaxy that struck the Milky Way’s center. Despite being classified as a dwarf galaxy, the Gaia Sausage possessed considerable mass, equivalent to more than 10 billion times that of the sun.
The ancient collision is now recognized as the most significant merger event in the Milky Way’s history, fundamentally reshaping the galaxy by creating its central bulge and significantly altering the stellar halo’s composition. The research findings were presented at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham earlier this week.
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