The Milky Way was dramatically flipped on its side after a head-on collision with a dwarf galaxy about 10 billion years ago, astronomers have revealed. This cataclysmic event, involving the so-called Gaia Sausage, reshaped our galaxy and set the stage for the solar system's eventual formation. Using supercomputer simulations, researchers at Durham University have uncovered how such impacts can reorient entire galactic disks.
The Collision That Flipped the Milky Way
About 10 billion years ago, a dwarf galaxy known as the Gaia Sausage slammed directly into the Milky Way. The impact was so powerful that it tore the interloper apart, absorbing its stars, but also caused the Milky Way's entire disk to flip by more than 90 degrees. This disc flip explains the oddly slow rotation of stars in the galaxy's halo today.
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Simulations Reveal the Mechanism
Lead researcher Kirill Batrakov and his team ran supercomputer simulations of Milky Way-like galaxies undergoing head-on collisions. The models showed that such collisions could indeed cause a gradual reorientation of the disk, taking at least a few hundred million years. The findings are being presented at the Royal Astronomical Society's National Astronomy Meeting.
Why the Halo Stars Rotate Slowly
Previous observations from the European Space Agency's Gaia mission revealed that stars in the Milky Way's disk orbit at about 220 km/s, while those in the stellar halo circle at just 25 km/s. The simulations now show that a disc flip from a major collision is the key factor creating this discrepancy.
| Star Population | Orbital Speed | Cause |
|---|---|---|
| Disk Stars | 220 km/s | Normal galactic rotation |
| Halo Stars | 25 km/s | Post-collision halo after disc flip |
Key Takeaways
- The Milky Way was flipped by a head-on collision with the Gaia Sausage dwarf galaxy 10 billion years ago.
- The disc flip explains the slow rotation of stars in the stellar halo.
- Supercomputer simulations by Durham University replicated the event, confirming the mechanism.
- Such collisions can take hundreds of millions of years to fully reorient a galaxy.
- The discovery sheds light on the violent early history of our galaxy.
FAQ
What caused the Milky Way to flip?
A head-on collision with a dwarf galaxy called the Gaia Sausage about 10 billion years ago flipped the Milky Way's disk by over 90 degrees.
How do scientists know this happened?
Researchers used supercomputer simulations of Milky Way-like galaxies and compared them with data from the Gaia space mission, which measured the slow rotation of halo stars.
How long did the disc flip take?
According to lead researcher Kirill Batrakov, the process likely took at least a few hundred million years to complete.
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