UAH researcher demonstrates Milky Way's last major galactic collision happened much more recently than previously thought

HUNTSVILLE, Ala. (JUN 6, 2024) – Dr. Tom Donlon, a postdoctoral researcher at The University of Alabama in Huntsville (UAH), a part of the University of Alabama System, is the lead author of a new paper published in the Monthly Notices of the Royal Astronomical Society that reveals the Milky Way Galaxy's last major collision occurred billions of years later than previously thought. Donlon worked with Dr. Heidi Newberg, professor of physics and astronomy, and her team at the Rensselaer Polytechnic Institute in Troy, NY, to conclude the Milky Way's last significant collision with another galaxy occurred no less than three billion years ago, rather than between eight and 11 billion years, as previously believed.

The data supporting these findings were collected via the European Space Agency's (ESA) Gaia spacecraft, a global space astrometry mission that is currently building the largest, most precise 3-D map of our galaxy ever attempted, surveying nearly two billion celestial objects throughout the Milky Way. Gaia monitors each of its target stars about 14 times per year, precisely charting their positions, distances, movements and changes in brightness. Donlon and Newberg used these observations to focus on the galaxy's so-called 'wrinkles,' features formed when other galaxies collide with the Milky Way, to derive their conclusions.

"We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way," Donlon explains. The new study served as his doctoral thesis at Rensselaer, advised by Newberg. "It's a sort of cosmic Benjamin Button, getting less wrinkly over time. By looking at how these wrinkles dissipate, we can trace when the Milky Way experienced its last big crash - and it turns out this happened billions of years later than we thought."

See animation, a dwarf galaxy colliding with the Milky Way

The team compared observations of the wrinkles with cosmological simulations to make the discovery. "For the wrinkles of stars to be as obvious as they appear in Gaia data, they must have joined us no less than three billion years ago - at least five billion years later than was previously thought," Newberg adds. "New wrinkles of stars form each time the stars swing back and forth through the center of the Milky Way. If they'd joined us eight billion years ago, there would be so many wrinkles right next to each other that we would no longer see them as separate features."

Colliding or interacting galaxies are galaxies whose gravitational fields generate a disturbance of one another. An example of a minor interaction is a satellite galaxy disturbing the primary galaxy's spiral arms, while a major interaction is a galactic collision, which may ultimately lead to a galactic merger.

Scientists originally presumed the Milky Way's last major collision was the Gaia-Sausage-Enceladus merger, dated at between eight and 11 billion years ago. These new findings suggest instead that the most recent major encounter was actually the Virgo Radial Merger, which crashed through the center of the Milky Way less than three billion years ago, evidenced as well by a large number of stars with unusual orbits thought to have been generated by the collision.

"Gaia is a hugely productive mission that's transforming our view of the cosmos," says Dr. Timo Prusti, project scientist for Gaia at ESA. "Results like this are made possible due to incredible teamwork and collaboration between a huge number of scientists and engineers across Europe and beyond."