The Milky Way once underwent a dramatic "major disc flip," according to a new study, with the galaxy's vast stellar disc changing orientation by more than 90 degrees and dragging the solar system along for the ride.
Researchers say the flip likely happened after a violent head-on collision with another galaxy. About 10 to 11 billion years ago, the Milky Way smashed into a massive dwarf galaxy known as Gaia-Sausage-Enceladus, or the Gaia Sausage. Scientists already knew that impact had sent billions of stars looping into sausage-shaped paths. The new study suggests it may have flipped the galaxy as well.
"We already know that the Milky Way had a massive head-on collision," said lead author Dr Kirill Batrakov, of Durham University. "So, we think that the Milky Way disc likely flipped in the past."
A puzzle in the outer halo
The discovery grew out of an effort to solve one of the Milky Way's long-standing mysteries. Most of the galaxy's stars sit in the flat spiral disc, a region about 120,000 light-years across and 1,000 light-years thick.
That disc is surrounded by the stellar halo, a sparsely populated region roughly 300,000 light-years across that extends over a million light-years in diameter at its outer limits. The halo is made up largely of stars pulled into the Milky Way from other galaxies through mergers over time.
What has puzzled scientists is that this halo rotates unusually slowly compared with those of other galaxies. The European Space Agency's Gaia mission found that a star in the outermost halo can take up to a billion years to complete one orbit of the galactic core. Until now, researchers had no explanation for why.
Testing the theory on 25 galaxies
To investigate, the researchers analysed the simulated evolution of 25 Milky Way-like galaxies. The findings were presented this week at the Royal Astronomical Society's National Astronomy Meeting in Birmingham.
Tracking the simulated galaxies over billions of years, the team found that the ones with the slowest-rotating stellar halos had two things in common: they had all experienced a head-on collision with another galaxy, and they had all undergone a major disc flip.
Because the Milky Way has both a slow-moving stellar halo and a head-on collision in its past, the researchers concluded it likely underwent a disc flip too.
A different solar system, once
The finding suggests the galaxy we see today may have looked and behaved very differently billions of years ago.
"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," Dr Batrakov said. "Meaning our 'stable' spot in the galaxy might not have been so stable for the Solar System's whole lifetime."
Because we live inside the Milky Way, scientists can study it in more detail than any other galaxy, making it a key testing ground for ideas about galaxy evolution. Dr Batrakov said the flip adds a new chapter to that story that must be accounted for when placing the Milky Way in the context of other galaxies. "What excites me the most is that this complex history can be reconstructed just from present-day observations," he said.
The link to dark matter
The researchers also found that the Milky Way's stellar halo is closely tied to the rotation of its dark matter halo, the invisible matter that makes up most of the galaxy's mass and holds its structure together.
That connection means understanding why the stellar halo rotates so slowly could help shed light on the nature of dark matter itself.
