The Milky Way may have undergone a dramatic change in orientation during its history, according to new research that helps explain a long-standing mystery about our galaxy.
Using supercomputer simulations of galaxies like the Milky Way, a team of astronomers from Durham University 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.
The research is being presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham .
Most of the Milky Way's stars are found in its flat spiral disc. Surrounding this is a much larger but much sparser stellar halo, made up mostly of stars that originally formed in smaller galaxies before being pulled into the Milky Way through galaxy mergers.
Observations from the European Space Agency's Gaia mission have shown that the Milky Way's stellar halo rotates very slowly, but astronomers have not understood why.
To investigate, the researchers analysed the evolution of 25 Milky Way-like galaxies in the Auriga suite of cosmological simulations, following their development over billions of years.
They found that galaxies with the slowest rotating stellar haloes shared two important features. They had experienced a major head-on merger with another galaxy, and they had also undergone a disc flip during their evolution.
"We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus (often simply called the Gaia Sausage). So, we think that the Milky Way disc likely flipped in the past," explained astronomer Kirill Batrakov, the lead researcher on the project.
The Gaia-Sausage-Enceladus was a massive dwarf galaxy that collided with and was absorbed by the early Milky Way about 10 to 11 billion years ago. This defining galactic merger was the largest event in the early history of the Milky Way and reshaped our galaxy, leaving billions of stars orbiting in highly elongated, sausage-shaped paths.
Identifying a past disc flip gives astronomers a new way to understand how the Milky Way assembled and may also provide indirect clues about the motion of its invisible dark matter halo.
"A disc 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.
The Milky Way is our best laboratory for testing how galaxies and dark matter evolve. A disc flip does not happen in every galaxy, so if its history included a major flip that has not been linked to observable features of our galaxy, then it offers astronomers clues about how similar galaxies formed.
"Because we live inside the Milky Way, we can study it in more detail than any other galaxy, which makes it a key testbed for understanding galaxies more broadly," Batrakov added.
"Finding that its disc 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. What excites me the most is that this complex history can be reconstructed just from present-day observations."
Batrakov's study also found that the rotation of the Milky Way's stellar halo is closely linked to the rotation of its dark matter halo, suggesting the two possibly evolved together as the galaxy grew by accreting smaller satellite galaxies.
The findings provide a possible explanation for one of the Milky Way's unusual features and offer new clues about how our galaxy formed and evolved over billions of years.
