When the Milky Way was young, a galactic catastrophe unfolded in the cosmic void—a violent collision with a smaller galaxy that reset our home's entire stellar disk. New research from the University of Barcelona and the Institute of Space Studies of Catalonia reveals that this cataclysmic merger, known as the Gaia-Sausage-Enceladus collision, occurred about 11 billion years ago, pushing back previous estimates by roughly a billion years.
For decades, astronomers have puzzled over the history of our galaxy's iconic rotating disk—that vast, pancake-shaped system of stars spiraling around our galaxy's center at over 220 kilometers per second. Understanding when this disk formed and took shape is crucial to comprehending our cosmic origins, since it holds the majority of the Milky Way's stars, including our sun. The clues lay hidden in the motions and ages of ancient stars themselves, which began moving in a coherent, rotating pattern at some point in the galaxy's early history—a moment scientists call the spin-up time.
The breakthrough came in 2018, when data from the European Space Agency's Gaia mission revealed an unusual population of stars with peculiar motions. These could only have been explained by a massive merger event roughly 10 billion years ago. That discovery launched a new era of investigation into what shaped our galaxy.
Now, using sophisticated computer simulations known as the Auriga simulations, researchers led by Matthew D. A. Orkney have cracked the puzzle. They modeled how galaxies similar to the Milky Way respond to ancient collisions, revealing a surprising insight: rotating stellar disks often form much earlier than previously thought, but can be completely or partially demolished by major galactic mergers. This means the moment when the Milky Way's disk appears to spin up doesn't necessarily mark the birth of that disk—it marks the galaxy's recovery from a catastrophic impact.
Applying these simulation insights to real observational data, the team refined the timing of the Gaia-Sausage-Enceladus merger to approximately 11 billion years ago. That same time window aligns remarkably with a sharp spike in star cluster formation throughout the Milky Way. This connection is no accident: galactic collisions compress enormous reservoirs of gas, triggering intense bursts of star formation in their wake. "Models of the Gaia-Sausage-Enceladus merger predict that a galactic firework should have followed the impact, raising star formation and fostering the formation of globular clusters," explains Chervin F. P. Laporte, a co-author at the French National Centre for Scientific Research. "This is the first time this link has been made."
The findings, published in the Monthly Notices of the Royal Astronomical Society, underscore a fundamental truth: understanding the Milky Way's structure requires understanding the ancient collisions that shaped it. While scientists cannot travel back in time to witness our galaxy's youth directly, they now have powerful new tools at their disposal. The James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array allow astronomers to observe the formation of similar galaxies in the distant universe, testing these theories against the cosmos itself. In this way, the story of our own galactic home continues to unfold.