BIRMINGHAM — Researchers at Durham University presented new supercomputer simulations at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham revealing that a collision with the Gaia Sausage dwarf galaxy approximately 10 billion years ago flipped the Milky Way on its side. The simulations indicate that this ancient impact reoriented the galaxy's disc by more than 90 degrees, fundamentally altering the structure of the system in which Earth resides.

The study utilized advanced computational models to trace the physical consequences of the merger, which involved a dwarf galaxy containing stars, gas, and dark matter amounting to more than 10 billion times the mass of the sun. This massive influx of material not only shifted the orientation of the galactic disc but also created the bulge at the center of the Milky Way. The findings build upon initial evidence of the ancient collision that was first identified in 2018 using observations from the Gaia mission.

The simulations show the distinct rotational dynamics within the galaxy that helped researchers model the event. Stars in the Milky Way's disc orbit the galactic center at approximately 220km (137 miles) per second, while stars in the stellar halo rotate at a significantly slower rate of approximately 25km per second. These differing velocities provided critical data points for reconstructing the timeline and mechanics of the collision.

Kirill Batrakov, the lead researcher on the project, provided context on the timescale required for such a dramatic structural shift to stabilize. "It probably takes at least a few hundred million years," Batrakov said. This period allowed the gravitational forces to settle and the new orientation to become the permanent configuration of the galaxy.

The research also offers perspective on future interactions between the Milky Way and other galactic bodies. While the Gaia Sausage collision was a transformative event, Batrakov noted that subsequent mergers will likely have a diminished effect due to the increased mass of the Milky Way and the smaller size of approaching galaxies. "It’s not as massive as the Gaia Sausage was, so it is not going to influence the Milky Way as much, and the Milky Way itself is far more massive now than it was back then," he said.

The presentation of these findings at the Royal Astronomical Society’s National Astronomy Meeting in Birmingham marks a significant step in understanding the formation history of the Milky Way. By identifying the Gaia Sausage merger as the cause of the galaxy's tilted disc and central bulge, the research provides a coherent explanation for several structural anomalies observed in current astronomical data. The use of supercomputer simulations allows scientists to test these hypotheses against the known velocities and distributions of stars in the disc and halo.

This work continues a line of inquiry that began with the 2018 identification of the collision evidence via the Gaia mission. Understanding how such massive mergers reorient galactic discs helps astronomers predict the long-term evolution of spiral galaxies and assess the stability of planetary systems within them. The detailed modeling of the 90-degree reorientation demonstrates the profound impact that even smaller satellite galaxies can have on the architecture of larger systems over billions of years.