The Milky Way may have undergone some cosmic gymnastics during its history with a dramatic disc flip, according to new research from our astronomers.
The research offers new clues about how our galaxy formed and evolved over billions of years.
Using supercomputer simulations of galaxies like the Milky Way, a team of astronomers from our Department of Physics found that galaxies with slowly rotating stellar haloes are more likely to have experienced a major 'disc flip', where the galaxy's disc changed its orientation by more than 90 degrees.
Durham PhD student Kirill Batrakov presented the study at the Royal Astronomical Society's National Astronomy Meeting in Birmingham, UK.
Milky Way and galaxies
Most of the Milky Way's stars are found in its flat spiral disc, while a much larger but sparser halo surrounds it. This halo is mainly made up of stars from smaller galaxies that were absorbed by the Milky Way through past mergers.
We already know that the Milky Way's stellar halo rotates very slowly, but astronomers have not understood why.
Researchers studied 25 Milky Way-like galaxies using simulations and found that the slowest-rotating haloes had two things in common: a major head-on galaxy merger and a disc flip.
Cosmic back flip
The Milky Way is known to have collided with a dwarf galaxy called Gaia-Sausage-Enceladus around 10 to 11 billion years ago, matching the first criterion of a major head-on merger. Given an extremely slow rotation of the Milky Way stellar halo, researchers also believe the Milky Way's disc has undergone a flip, changing the paths of many stars, possibly including our Sun.
A disc flip 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.
The Milky Way is the best laboratory for studying how galaxies and dark matter evolve. Because disc flips do not happen in every galaxy, evidence that the Milky Way experienced one could give astronomers valuable clues about how our galaxy, and others like it, formed over time.
