A head-on collision with another galaxy billions of years ago may have drastically reoriented the Milky Way’s disk. New supercomputer simulations reveal how this violent merger created a spiral galaxy that appears surprisingly normal today.

Batrakov and his team deployed supercomputers to simulate galaxies like the Milky Way. Their findings suggest galaxies with slowly rotating stellar halos underwent large “disk flips,” leading to significant changes in orientation. Image credit: Batrakov et al.

Most stars in the Milky Way reside in a flat spiral disk, surrounded by a larger, sparser stellar halo, primarily made up of stars that formed in smaller galaxies and were drawn into the Milky Way through cosmic mergers.

ESA’s Gaia mission observations reveal that the Milky Way’s stellar halo rotates slowly, leaving astronomers puzzled about the reasons behind this phenomenon.

To investigate further, Dr. Kirill Batrakov from Durham University and his team studied the evolution of 25 Milky Way-like galaxies in the Auriga series of cosmological simulations, tracking their development over billions of years.

The researchers discovered that galaxies with the slowest rotating stellar halos shared two crucial traits: they experienced major head-on mergers and disk flips during their evolution.

“The Milky Way has undergone a significant head-on collision with a galaxy known as Gaia Sausage Enceladus, often referred to as Gaia Sausage,” Dr. Batrakov stated.

“This suggests it’s quite plausible that the Milky Way’s disk has flipped in the past.”

Gaia Sausage Enceladus, a massive dwarf galaxy, collided with and merged into the early Milky Way around 10 to 11 billion years ago, marking a pivotal moment in our galaxy’s history and contributing to the formation of elongated, sausage-shaped stellar orbits.

Understanding past disk flips could provide astronomers with insights into the Milky Way’s formation and offer indirect clues about the behavior of its dark matter halo.

“The flipping of the disk suggests that many stars in the Milky Way, including our Sun, once followed very different orbits than they do today. This indicates our ‘stable’ position in the galaxy may not have been stable throughout our solar system’s history,” Dr. Batrakov noted.

Disk flips are not common in all galaxies. If the Milky Way’s history includes substantial flips without observable signs, it may shed light on the formation of similar galaxies.

“Since we inhabit the Milky Way, we can delve into its intricacies more than any other galaxy, making it a vital testing ground for understanding galaxies comprehensively,” Dr. Batrakov emphasized.

“The revelation that the disk has flipped adds a significant chapter to its history, which needs to be considered when placing the Milky Way in the broader context of other galaxies.”

“What excites me the most is that this intricate history can be pieced together solely from current observations.”

The research team also determined that the rotation of the Milky Way’s stellar halo is closely tied to its dark matter halo’s rotation, indicating that both may have evolved jointly as the Milky Way amassed smaller satellite galaxies.

“This finding offers a potential explanation for one of the Milky Way’s distinctive features and provides new insights into our galaxy’s formation and evolution over billions of years,” the researchers concluded.

The researchers presented their findings at the National Astronomical Conference 2026 (NAM2026), held this week in Birmingham, England.

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Kirill Batrakov et al. Discussing the slow rotation of the Milky Way’s stellar halo. NAM2026