Milky Way flipped over 90 degrees after ancient collision, study finds

The Milky Way's stellar disc was dramatically reoriented — flipping more than 90 degrees — following a head-on collision with a massive dwarf galaxy roughly 10 to 11 billion years ago, according to new research from Durham University presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.miragenews+1

A Long-Standing Mystery

The finding offers an explanation for one of galactic astronomy's persistent puzzles: why the Milky Way's stellar halo rotates unusually slowly. Using the Auriga suite of cosmological supercomputer simulations, researchers tracked the evolution of 25 Milky Way-like galaxies over billions of years. They found that galaxies with the slowest-rotating stellar haloes shared two features — they had experienced a major head-on merger and undergone a disc flip during their evolution.phys+1

"We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus," said Kirill Batrakov, the lead researcher on the project. "So, we think that the Milky Way disk likely flipped in the past."news.yahoo+1

The Gaia-Sausage-Enceladus was a dwarf galaxy absorbed by the early Milky Way in what is considered the largest merger event in our galaxy's early history. The collision left billions of stars orbiting in highly elongated, sausage-shaped paths and reshaped the galaxy's structure.simonsfoundation+1

Implications for Our Solar System

The disc flip implies that most of the Milky Way's stars, including possibly our own Sun, once moved on very different trajectories than they do today. "A disk flip also means most of the Milky Way's stars once moved on very different trajectories than they do today — possibly even our own Sun, meaning our 'stable' spot in the galaxy might not have been so stable for the solar system's whole lifetime," Batrakov said.phys+1

Connecting Dark Matter and Stellar Haloes

The study also found that the rotation of the Milky Way's stellar halo is closely linked to that of its dark matter halo, suggesting the two evolved together as the galaxy grew by consuming smaller satellite galaxies.phys

"Finding that its disk flipped adds a new chapter to that story, one we must account for when placing the Milky Way in a broader context of other galaxies," Batrakov said. "What excites me the most is that this complex history can be reconstructed just from present-day observations."phys