Astronomers using supercomputer simulations have revealed that the Milky Way experienced a 90-degree disk flip roughly 10 billion years ago, triggered by a head-on collision with a massive dwarf galaxy known as Gaia-Sausage-Enceladus.
The history of our galaxy is far more violent than its serene, starry spiral arms suggest. Long before the solar system formed, the Milky Way was struck directly by an incoming interloper with a mass exceeding 10 billion times that of our sun. The ensuing chaos tore the dwarf galaxy apart, absorbing its stars into a diffuse stellar halo, but the immense impact also fundamentally reshaped the architecture of our cosmic home.
Decoding the Slow Rotation of the Stellar Halo
For years, astronomers grappled with a persistent astronomical puzzle concerning the galaxy’s outer reaches. While stars within the Milky Way’s primary disk orbit the galactic center at a brisk pace of about 220 kilometers per second, the stars residing in the surrounding stellar halo rotate at a sluggish 25 kilometers per second. Data from the European Space Agency’s Gaia mission confirmed that this sparse stellar halo possesses only a weak net rotation, leaving researchers searching for a physical mechanism to explain the extreme disparity.
To investigate the mystery, a research team led by Kirill Batrakov at Durham University turned to supercomputer simulations. Analyzing the evolution of 25 Milky Way-like galaxies within the Auriga cosmological zoom-in simulation suite, the team tracked how galactic interactions alter stellar motion over billions of years. The models accounted for gravity, dark matter, black holes, and supernovae from a redshift range covering the last 11 billion years.
The Mechanics of a 90-Degree Galactic Reorientation
The simulations revealed a striking pattern: galaxies that developed slow-moving stellar haloes consistently shared a specific evolutionary history. They formed early, experienced a head-on collision like the Milky Way’s encounter with the Gaia-Sausage-Enceladus galaxy, and subsequently underwent a dramatic disk flip.
According to the findings presented at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham, this reorientation was not an instantaneous flash. The disk flip likely took anywhere from 150 million years to over a billion years to unfold as the galaxy adjusted to the impact.
“We already know that the Milky Way had a massive head-on collision in the past with a galaxy known as Gaia-Sausage-Enceladus, so, we think that the Milky Way disk likely flipped in the past.”
Kirill Batrakov, Durham University
The collision threw stars from the shredded dwarf galaxy onto radical, elongated sausage-shaped orbits that were heavily misaligned with the host disk. When the galactic disk itself subsequently flipped by more than 90 degrees, the surrounding halo failed to reorient immediately. This prolonged synchronization lag resulted in the slow, misaligned halo rotation observed today.
Implications for the Solar System and Galactic Context
Because the collision and subsequent flip occurred roughly 10 billion years ago—long before the birth of the Sun—our solar system missed the most chaotic phases of the upheaval. However, researchers point out that the reorganization fundamentally altered the trajectories of the galaxy’s stellar population.
“A disk flip means that most of the Milky Way’s stars once moved on very different trajectories than they do today, possibly even our sun, meaning our ‘stable’ spot in the galaxy might not have been so stable for the solar system’s whole lifetime.”
Kirill Batrakov, Durham University
By connecting present-day observations from the Gaia observatory with cosmological models, astronomers have gained a clearer framework for interpreting galaxy evolution across the universe. Because scientists can study the Milky Way in far greater detail than any external galaxy, establishing its precise history serves as an essential baseline for understanding galactic structures everywhere.
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