The Milky Way's last major act of galactic cannibalism was surprisingly recent

New findings from the Gaia space telescope indicate the Milky Way may have cannibalized a small galaxy not too long ago, cosmically speaking. In fact, the last major collision between our galaxy and another seems to have occurred billions of years later than previously suspected.

The Milky Way has been long understood to have grown via a series of violent collisions, which see smaller galaxies ripped apart by the immense gravitational influence of our solar system's spiral home. These collisions distribute stars from the devoured galaxy across the halo that surrounds the main disk of the Milky Way and its distinctive spiral arms. These bouts of galactic cannibalism also send "wrinkles" rippling through the Milky Way that affect different "families" of stars, with different origins, in different ways. 

With its ability to precisely pinpoint the position and motion of over 100,000 stars local to the solar system within the full catalog of stellar bodies in monitors, Gaia aims to retell the history of the Milky Way by counting its wrinkles.  

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"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," Thomas Donlon, study team leader of the Rensselaer Polytechnic Institute and University of Alabama scientist, said in a statement. "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."

These galactic wrinkles were only discovered by Gaia in 2018; this marks the first time they have been extensively researched to reveal the timing of the collision that created them.

Halo stars moving in strange ways

The halo of our galaxy is populated with stars that have strange orbits, with many of these believed to be the "leftovers" of galaxies the Milky Way once devoured.

Many of those stars are believed to be the wreckage of the so-called "last major merger," referring to the last time the Milky Way experienced a significant collision with another galaxy. Scientists think this final major collision may have involved a massive dwarf galaxy, and the event is known as the Gaia-Sausage-Enceladus (GSE) merger. It is thought to have infused the Milky Way with stars on orbits that bring them close to the Galactic Center. The GSE event is thought to have happened between eight and 11 billion years ago when the Milky Way was in its infancy.

Since 2020, Thomas and his team have been comparing the Milky Way's wrinkles to simulations of how galactic collisions and mergers could have created them. However, the Gaia observations of these strangely orbiting stars — released as part of the space telescope's Data Release 3 in 2022 — indicate these odd stellar bodies could have been deposited by a different merger event.

"We can see how the shapes and number of wrinkles change over time using these simulated mergers. This lets us pinpoint the exact time when the simulation best matches what we see in real Gaia data of the Milky Way today — a method we used in this new study, too," explained Donlon. "By doing this, we found that the wrinkles were likely caused by a dwarf galaxy colliding with the Milky Way around 2.7 billion years ago. We named this event the Virgo Radial Merger."

"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," team member Heidi Jo Newberg, also of Rensselaer Polytechnic Institute, said. "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 recent examination of Gaia's observations calls into question whether a massive ancient merger in the early history of the Milky Way is really needed to explain the strange orbits of some stars in the galactic. It also casts doubt on all the stars previously associated with the GSE merger.

"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," Donlon said. "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."

The team also thinks that the Virgo Radial Merger brought to our galaxy a family of other small dwarf galaxies and star clusters, all of which would also have been devoured by the Milky Way at around the same time.

Future investigation and data from Gaia could show if any objects previously associated with the GSE event are actually connected to the more recent Virgo Radial Merger.

This new research is the latest in a treasure trove of results emerging from Gaia data that are rewriting the history of the Milky Way. 

Such cosmic revisionism has been made possible thanks to Gaia's unique ability to explore a vast number of stars over Earth, allowing the space telescope to compile an unrivaled map of the positions, distances and motions of around 1.5 billion stars thus far.

"The Milky Way's history is constantly being rewritten, in no small part, thanks to new data from Gaia," Donlon concluded. "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."

The team's research was published in May in the journal Monthly Notices of the Royal Astronomical Society.