Milky Way Simulated to Have Flipped Disk After Ancient Dwarf Galaxy Collision

For years, astronomers have puzzled over the strange, lethargic movement of the stars in the Milky Way’s halo. While stars within our galaxy’s flat, spiral disk whip around the galactic center at approximately 220 kilometers per second, the stars in the sparse, spherical halo circle at a mere 25 kilometers per second. New research suggests this disparity is not a mystery of gravity alone, but a scar left by a violent cosmic history.

The Gaia-Sausage-Enceladus Collision

The event centers on a massive, ancient merger. Approximately 10 to 11 billion years ago, a dwarf galaxy—often called the Gaia Sausage—plowed directly into the early Milky Way. The impact was so severe that it tore the smaller galaxy apart, scattering its stars into the elongated, “sausage-shaped” orbits that astronomers first mapped in 2018 using data from the European Space Agency’s Gaia mission.

While the merger is recognized as the most significant event in the early history of our galaxy, the new simulation work indicates that the consequences went beyond mere absorption. The gravitational torque from this face-on strike likely forced the Milky Way’s disk to tilt, eventually flipping its orientation by more than 90 degrees over several hundred million years.

We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus (often simply called the Gaia Sausage).

Kirill Batrakov, lead researcher at Durham University

Decoding the Stellar Halo with Auriga Simulations

To test the theory that a “disk flip” explains the slow stellar halo, the team analyzed 25 Milky Way-like galaxies within the Auriga suite of cosmological simulations. These simulations allow researchers to track the evolution of galaxies over billions of years, providing a high-resolution window into the past.

The results showed a clear pattern: galaxies that experienced both a head-on merger and a subsequent disk flip consistently ended up with the same slow-rotating stellar haloes observed in our own galaxy. This link provides a powerful new tool for understanding galactic formation. By reconstructing this history, astronomers can better infer the motion of the invisible dark matter halo that envelopes our galaxy.

Implications for the Solar System’s Stability

The findings presented at the National Astronomy Meeting in Birmingham suggest that our current “stable” position in the galaxy is a relatively recent development in the grand scheme of cosmic time. If the disk did indeed tilt by 90 degrees, the stars—and potentially the precursors to our own solar system—would have traversed vastly different trajectories than they do today.

Despite the dramatic nature of these events, the ScienceAlert report notes that the process was not cataclysmic in the way a fast-motion movie might suggest.

Reconstructing History from Present Observations

The ability to map these ancient events using only modern data is a point of particular excitement for the research team. By studying the Milky Way as a nearby “testbed”, astronomers hope to translate these findings to other galaxies, building a more comprehensive understanding of how galactic disks and dark matter halos co-evolve.

As Batrakov noted, this research adds a new chapter to the story of our home galaxy. The team’s findings suggest that the complex, turbulent history of the Milky Way can be read in the current movements of its stars, turning the night sky into a map of ancient, cosmic gymnastics.