🌌 A collision may have flipped the Milky Way

The Milky Way may have changed its orientation after a head-on collision with a dwarf galaxy about 10 billion years ago. Simulations presented at the Royal Astronomical Society's annual meeting indicate that its disk could have tilted by more than 90 degrees.

The galactic disk gathers the majority of the Milky Way's stars in a flattened structure. The Sun is located in this region, which also contains the spiral arms and the galactic center. However, its current shape does not directly reveal the ancient upheavals it has undergone.

The researchers focused on the rotation of the stellar halo. This vast envelope surrounds the disk and contains stars that move much more slowly around the galactic center. Their particular motion bears the imprint of an ancient merger between two galaxies.

The dwarf galaxy involved, Gaia-Enceladus, is colloquially called Gaia-Sausage. Its stars were identified by their elongated trajectories and certain shared characteristics. They are thought to be the remnants of a galaxy absorbed by the Milky Way early in its history.

To understand the consequences of this encounter, the team used numerical simulations of galaxies. The calculations show that a nearly head-on collision can cause a galaxy's disk to pivot. This tilting unfolds over several hundred million years.

The results do not mean the Milky Way was flipped like a solid object. Stars follow individual orbits within a vast gravitational system. The collision gradually modified the overall organization of the disk and the direction of its angular momentum.

This evolution could explain why the Milky Way's halo rotates significantly slower than its disk. It also provides a framework for interpreting the current distribution of stars from the absorbed dwarf galaxy.

The study remains based on simulations and the interpretation of stellar motions. Further observations will need to refine the chronology of the collision and its exact influence.