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Gaia: The Milky Way's last major collision happened surprisingly recently

Until now it was thought that the last big collision was 10 billion years ago, and it turns out that there was one 3 billion years ago as well

Simulation of a smooth halo of stars around the Milky Way. Figure: ESA
Simulation of a smooth halo of stars around the Milky Way. Figure: ESA

Our galaxy has collided with many galaxies during its existence. Now, the European Space Agency's (ESA) Gaia space telescope reveals that the last collision between them happened billions of years later than we thought.

The Milky Way grew over time as other galaxies approached, collided, ripped apart and were swallowed up by our galaxy. Each collision caused wrinkles that are still spreading through different families of stars, affecting their movement and behavior in space.

One of Gaia's goals is to unravel the history of our galaxy by studying these wrinkles—something it does by determining the positions and motions of more than 100,000 nearby stars, a tiny fraction of the two billion sources it observes.

Revealing the history of the Milky Way through a stellar halo

"We get more wrinkled as we age, but our research shows that the opposite is true for the Milky Way. It's sort of a cosmic version of Benjamin Button, getting less wrinkled over time," says Thomas Donlon of Rensselaer Polytechnic Institute and the University of Alabama in Huntsville, USA, and lead author of the new Gaia study. "By looking at the way these wrinkles dissipate over time, we can track when the Milky Way experienced its last major collision — and it turns out it happened billions of years later than we thought."

These galactic wrinkles were only discovered by Gaia in 2018. This study is the first to precisely determine the timing of the collision that created the wrinkles, by comparing observations with cosmological simulations.

strange movements

The halo of the Milky Way contains a large group of stars with unusual orbits, many of which are thought to have been added to our galaxy during an event astronomers call "the last great merger." As the name suggests, this is the last time our galaxy experienced a significant collision with another galaxy - which was probably a giant dwarf galaxy flooding the Milky Way with stars that pass very close to the center of our galaxy.

Scientists have dated this merger to between eight and eleven billion years ago, when the Milky Way was in its youth, and is known as the GSE. But the data from Gaia – released as part of the telescope's 2022 data delivery – now suggests that another merger may have provided the strangely moving stars.

"For the stars' wrinkles to be as clear as they appear in the Gaia data, they would have had to join us less than three billion years ago—at least five billion years later than we previously thought," adds co-author Heidi Jo Newberg, also of Rensselaer Polytechnic Institute. . “New star wrinkles are created every time the stars oscillate back and forth through the center of the Milky Way. If they had joined us eight billion years ago, there would be so many wrinkles side by side that we wouldn't see them as separate features."

The finding suggests that instead of these stars coming from the ancient GSE merger, they came from a more recent event called the Virgo Radial Merger, which occurred less than three billion years ago.

Rewriting history

There is evidence of GSE mergers occurring far back in the history of the Milky Way. However, recent work is challenging whether a massive ancient merger is indeed necessary to explain the properties of the Milky Way as we see it today, and whether all the stars originally identified with GSEs are from the same merger event.

In 2020, Thomas led the study that identified kinks in stars in the Milky Way and compared them to simulations of different possible mergers. "We can see how the shapes and number of wrinkles change over time using these simulated mergers. This allows us to determine the exact time when the simulation best matches what we see in the real Gaia data of the Milky Way today – a method we also used in this new study,” says Thomas. "In doing so, we found that the wrinkles were probably caused by a dwarf galaxy that collided with the Milky Way around 2.7 billion years ago. We called this event a virgin radial merger.”

Since then, Thomas and his colleagues have studied this merger further, refining the idea that many of the strangely moving stars and debris in the Milky Way's inner halo were brought into our galaxy by a more recent galactic collision than the GSE. They also clarified that the stars originally identified with the GSE may have resulted from multiple mergers, some of them ancient.

"The history of the Milky Way is being rewritten all the time, thanks to new data from Gaia," adds Thomas. "Our picture of the Milky Way's past has changed dramatically even from a decade ago, and I think our understanding of these mergers will continue to change rapidly.

"This finding - that a large part of the Milky Way joined us only during the last billion years - is a big change from what astronomers thought until now. Many popular models and ideas about how the Milky Way grew would expect a recent head-on collision with a dwarf galaxy of this mass to be very rare.”

Apparently the Virgo radial merger brought with it a family of other small dwarf galaxies and star clusters, all of which joined the Milky Way around the same time. Future studies will reveal which of these small objects, previously thought to be related to the ancient GSE merger, are actually related to the more recent Virgo radial merger.

Amazing collaboration

This finding joins the stream of results from Gaia that are rewriting the history of our cosmic home. The space telescope is uniquely positioned to study the vast variety of stars in our sky, and has managed to compile an unprecedented dataset of the positions, distances and movements of some 1.5 billion stars to date.

"Gaia is an extremely fruitful mission that is changing our view of the universe," says Timo Frosti, Gaia project scientist at ESA. "Such results are made possible by incredible teamwork and collaboration between a huge number of scientists and engineers across Europe and beyond."

"This finding improves what we know about the many complicated events that shaped the Milky Way, and helps us better understand how galaxies form and crystallize - and especially our home galaxy."

for the scientific article

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