Ancient Collision with 'Sausage' Galaxy Reshaped the Milky Way

The Milky Way was fundamentally transformed billions of years ago by a violent head-on collision with a massive dwarf galaxy. This ancient merger, the last major event of its kind in the galaxy's history, effectively rebuilt the structural foundation of our galactic home.

Between 8 and 10 billion years ago, a dwarf galaxy known as Gaia-Enceladus—or the "Sausage" galaxy—slammed into the young Milky Way. The intruder was substantial, with a total mass of gas, stars, and dark matter exceeding 10 billion solar masses. The impact was so severe that it ripped the dwarf galaxy to shreds. According to Vasily Belokurov of the University of Cambridge, the collision left the dwarf's stars moving on highly radial orbits, which he described as being "like needles."

This discovery was made possible by the European Space Agency's Gaia satellite, which mapped the positions and trajectories of millions of stars. By plotting these velocities, researchers identified a distinct pattern. Wyn Evans of Cambridge's Institute of Astronomy noted that the "sausage shape" of the data jumped out at the team, giving the event its colloquial name.

The Galactic Aftermath

The merger acted as a primary architect for the Milky Way's current anatomy. The collision is credited with creating the galaxy's inner bulge and the surrounding stellar halo. Furthermore, the impact likely fractured or "puffed up" the Milky Way's existing disk, a disruptive event that forced the disk to eventually regrow. The Sausage galaxy also contributed physical assets to the Milky Way, bringing at least eight globular clusters along with it during the merger.

Why the Merger Matters

For astronomers, the wreckage of Gaia-Enceladus serves as a fossil record of early galactic evolution. By analyzing these stars, scientists can reconstruct how spiral galaxies evolve over eons. One key finding is that stars formed after the collision contain less metal than those that predated the merger, providing a chemical timeline of the event. While the Milky Way continues to accrete smaller objects, such as the Sagittarius dwarf galaxy, the Gaia-Enceladus event was significantly more transformative due to its sheer scale.

Future Observations

Researchers will continue to use Gaia data to refine the timeline of this merger and its role in the formation of the thick disk. While the primary structure of the collision is understood, astronomers remain focused on mapping the remaining debris of the Sausage galaxy to determine exactly how much of the Milky Way's current mass originated from this ancient intruder.