Astronomers have determined that the Milky Way underwent a massive structural transformation approximately 10 billion years ago when it collided head-on with a dwarf galaxy known as the Gaia Sausage. Simulations from Durham University researchers indicate this cataclysmic event likely flipped the Milky Way’s disc by more than 90 degrees, fundamentally reshaping the galaxy’s orientation and stellar halo.
The Gaia Sausage Collision and Galactic Reorientation
The collision with the Gaia Sausage serves as the Milky Way’s last major merger, according to research presented at the Royal Astronomical Society’s National Astronomy Meeting. While the Milky Way consumed the intruder, the impact triggered a “disc flip” that permanently altered the galaxy’s configuration. Lead researcher Kirill Batrakov of Durham University notes that this reorientation process likely spanned several hundred million years.
Evidence for this event was first identified in 2018 through data from the European Space Agency’s (ESA) Gaia mission. Astronomers discovered stars moving on distinct, sausage-shaped orbits, which acted as a “smoking gun” for the merger. These stars originated from the dwarf galaxy, which contained gas, stars, and dark matter totaling more than 10 billion times the mass of the sun.
Did you know? The stellar halo—the sparse, ball-shaped cloud of stars surrounding our galaxy—rotates at only 25km per second, significantly slower than the 220km per second speed of stars within the Milky Way’s disc.
Solving the Stellar Halo Velocity Mystery
For years, astronomers struggled to explain why stars in the galactic halo rotate so much slower than those in the disc. By utilizing supercomputer simulations, the Durham team demonstrated that head-on collisions with galaxies the size of the Gaia Sausage naturally produce this velocity disparity. The resulting disc flip explains why the halo contains slow-moving stars that were scattered during the ancient merger.
This discovery confirms that the Gaia Sausage was not merely a minor visitor but a defining architect of our galaxy. The merger created the bulge at the center of the Milky Way and dominates the composition of the current stellar halo.
Future Galactic Mergers and Cosmic Evolution
While the Milky Way is currently absorbing the Sagittarius dwarf galaxy, researchers suggest this event will have a minimal impact compared to the Gaia Sausage collision. The Sagittarius galaxy is significantly smaller, and the Milky Way is now far more massive than it was 10 billion years ago, making it more resistant to structural upheaval.
Looking further ahead, the next significant transformation is expected when the Large Magellanic Cloud merges with the Milky Way. According to researchers, this event is likely several billion years away. These observations provide a roadmap for understanding how galaxies evolve from early, chaotic mergers into the structured discs observed in the modern universe.
Frequently Asked Questions
- What is the Gaia Sausage? It is a dwarf galaxy that collided with the Milky Way 10 billion years ago, leaving behind stars on unique, elongated orbits.
- Why did the Milky Way flip? The head-on impact of the massive Gaia Sausage forced the Milky Way’s disc to reorient by more than 90 degrees.
- Is the Milky Way still colliding with other galaxies? Yes, the galaxy is currently merging with the Sagittarius dwarf galaxy, though it is much smaller than the Gaia Sausage.
- How fast do stars in the Milky Way disc move? Stars in the disc orbit the galactic center at approximately 220km per second.
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