A head-on collision with another galaxy billions of years ago may have dramatically reoriented the Milky Way’s disk, according to new supercomputer simulations that reveal how violent mergers can leave spiral galaxies looking surprisingly normal today.
Using supercomputer simulations of galaxies like the Milky Way, Batrakov et al. found that galaxies with slowly rotating stellar haloes are more likely to have experienced a major ‘disk flip,’ where our Galaxy’s disk changed its orientation by more than 90 degrees. Image credit: Batrakov et al.
Most of the Milky Way’s stars are found in its flat spiral disk.
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 ESA’s Gaia mission have shown that the Milky Way’s stellar halo rotates very slowly, but astronomers have not understood why.
To investigate, Dr. Kirill Batrakov and his colleagues from Durham University analyzed 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 disk 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,” Dr. Batrakov said.
“So, we think that the Milky Way disk likely flipped in the past.”
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 disk 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 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,” Dr. Batrakov said.
A disk 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,” Dr. Batrakov said.
“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.”
“What excites me the most is that this complex history can be reconstructed just from present-day observations.”
The astronomers 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,” they said.
The researchers presented their results this week at the 2026 National Astronomy Meeting (NAM2026) in Birmingham, the United Kingdom.
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Kirill Batrakov et al. Why is the Milky Way stellar halo slowly rotating? NAM2026
