Milky Way flipped >90° after dwarf merger, study finds

Astronomers have proposed that the Milky Way galaxy may have slowly reoriented itself by more than 90 degrees billions of years ago, following a colossal collision with a smaller galaxy. The findings, presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham, suggest this dramatic "disc flip" could explain a long-standing mystery about the unusually slow rotation of stars in the galaxy's outer halo.

The research, led by Kirill Batrakov of Durham University, used supercomputer simulations to model the evolution of Milky Way-like galaxies. The team found that galaxies with stellar haloes rotating as slowly as the Milky Way's often shared two key events in their past: a major head-on merger and a subsequent reorientation of their galactic disc. The merger implicated is with a dwarf galaxy known as Gaia-Sausage-Enceladus, often called the Gaia Sausage, which is believed to have smashed into the Milky Way roughly 8 to 11 billion years ago.

This ancient collision, first identified by astronomers in 2018 using data from the European Space Agency's Gaia mission, is considered the Milky Way's last major merger. It fundamentally reshaped our galaxy,...