The Milky Way’s most recent major merger occurred billions of years later than previously believed

The discovery was made possible by ESA’s Gaia spacecraft, which is mapping more than a billion stars across the Milky Way galaxy and beyond, tracking their motions, brightness, temperature, and composition.

The Milky Way galaxy has grown over time as other galaxies have approached, collided, been torn apart, and been swallowed up.

Each collision still sends ripples through different groups of stars, influencing their movements and behavior in space.

One of Gaia’s goals is to study these wrinkles to unravel the history of our Milky Way galaxy. It does this by pinpointing the positions and motions of more than 100,000 stars close to Earth, a tiny fraction of the roughly 2 billion objects it observes.

“As we age, we tend to get more wrinkles, but our research shows that the opposite is true in the Milky Way – it’s like a cosmic Benjamin Button, and it gets less wrinkled over time,” said Dr. Thomas Donlon, an astronomer at Rensselaer Polytechnic Institute and the University of Alabama.

“By looking at how these wrinkles fade over time, we can trace when the Milky Way last experienced a major collision — and it turns out this happened billions of years later than we thought.”

The Milky Way’s halo contains many stars with unusual orbits, many of which are thought to have been incorporated into the galaxy in an event that astronomers call the last great merger.

As the name suggests, this is the last time the Milky Way has experienced a significant collision with another galaxy, which is proposed to have been a giant dwarf galaxy that smothered the Milky Way with stars passing very close to the center of the Milky Way.

Astronomers estimate that the merger occurred between 8 and 11 billion years ago, when the Milky Way was still in its infancy, and is known as Gaia-Sausage-Enceladus.

But data from Gaia’s Data Release 3 suggests that another merger could have resulted in the unusually behaving star.

“For the stellar wrinkles to be as clear as we see in the Gaia data, the stars would have had to have appeared on Earth less than 3 billion years ago — at least 5 billion years later than previously thought,” said Dr. Heidi Jo Neuberg, also of Rensselaer Polytechnic Institute.

“Every time a star passes back and forth through the center of the Milky Way, a new stellar wrinkle forms.”

“If they had merged with us 8 billion years ago, there would have been so many wrinkles next to each other that we wouldn’t be able to see them as separate features.”

This discovery suggests that these stars did not result from the ancient Gaia-Sausage-Enceladus merger, but must have arisen from a more recent event called the Virgo radial merger, which occurred less than 3 billion years ago.

“The history of the Milky Way is currently being constantly rewritten, thanks in large part to new data from Gaia,” Dr. Donlon said.

“Our image of the Milky Way’s past has changed dramatically since even 10 years ago, and I think our understanding of these mergers will continue to change rapidly.”

“This finding that most of the Milky Way galaxy joined Earth within the last few billion years is quite different from what astronomers previously thought.”

“Many prevailing models and ideas about the growth of the Milky Way predict that a recent head-on collision with a dwarf galaxy of this mass would be extremely rare.”

“The Virgo radial merger likely pulled in a group of other small dwarf galaxies and star clusters, all of which joined the Milky Way at about the same time.”

“Future exploration will reveal which of these small objects previously thought to be related to the ancient Gaia sausage Enceladus are in fact related to the recent Virgo radial merger.”

of Investigation result Appears in Monthly Bulletin of the Royal Astronomical Society.

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Thomas Donlon otherThe year is 2024. The remains of the “last great merger” are dynamically young. MNRAS 531(1):1422-1439; doi:10.1093/mnras/stae1264