Milky Way’s Last Major Merger Occurred Billions of Years Later than Previously Thought

This discovery was made possible by ESA’s Gaia spacecraft, which is mapping more than a billion stars throughout the Milky Way Galaxy and beyond, tracking their motion, luminosity, temperature, and composition.

The Milky Way has grown over time as other galaxies have approached, collided with, and been torn apart and consumed by our Galaxy.

Each collision triggered wrinkles that still ripple through different families of stars, affecting how they move and behave in space.

One of Gaia’s aims is to unravel the history of the Milky Way by studying these wrinkles — something it’s doing by pinpointing the positions and motions of over 100,000 stars near to our own, a tiny fraction of the about two billion sources it observes.

“We get wrinklier as we age, but our work reveals that the opposite is true for the Milky Way. It’s a sort of cosmic Benjamin Button, getting less wrinkly over time,” said Dr. Thomas Donlon, an astronomer with Rensselaer Polytechnic Institute and the University of Alabama.

“By looking at how these wrinkles dissipate over time, 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.”

The Milky Way’s halo contains a large group of stars with unusual orbits, many of those thought to have been adopted into our Galaxy during an event that astronomers call the last major merger.

As the name suggests, this is the last time the Galaxy experienced a significant collision with another galaxy — proposed to be a massive dwarf galaxy that flooded the Milky Way with stars that pass very close to the Galactic center.

Astronomers had dated this merger to between eight and eleven billion years ago, when the Milky Way was in its infancy, and it is known as Gaia-Sausage-Enceladus.

But the data from Gaia’s Data Release 3 now suggests that another merger may have delivered the unusually moving stars.

“For the wrinkles of stars to be as clear as they appear in Gaia data, they must have joined us less than three billion years ago — at least five billion years later than was previously thought,” said Dr. Heidi Jo Newberg, also of Rensselaer Polytechnic Institute.

“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.”

The finding suggests that rather than these stars originating from the ancient Gaia-Sausage-Enceladus merger, they must have come from a more recent event dubbed the Virgo Radial Merger, which took place less than three billion years ago.

“The Milky Way’s history is constantly being rewritten at the moment, in no small part thanks to new data from Gaia,” Dr. Donlon said.

“Our picture of the Milky Way’s past has changed dramatically from even a decade ago, and I think our understanding of these mergers will continue to change rapidly.”

“This result — that a large portion of the Milky Way only joined us within the last few billion years — is a big change from what astronomers thought up until now.”

“Many popular models and ideas about how the Milky Way grows would expect a recent head-on collision with a dwarf galaxy of this mass to be very rare.”

“It’s likely that the Virgo Radial Merger brought in a family of other small dwarf galaxies and star clusters with it, which would have all joined the Milky Way at around the same time.”

“Future exploration will reveal which of these smaller objects that were previously thought to be related to an ancient Gaia-Sausage-Enceladus are actually related to a more recent Virgo Radial Merger instead.”

The findings appear in the Monthly Notices of the Royal Astronomical Society.

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Thomas Donlon et al. 2024. The debris of the ‘last major merger’ is dynamically young. MNRAS 531 (1): 1422-1439; doi: 10.1093/mnras/stae1264