Recent Cosmic Collision Shakes Up Milky Way History

Evidence of the Milky Way’s past is encoded in wrinkles of stars, whose positions and movements have shifted as our galaxy has interacted with other galaxies, sometimes violently. Now, a team of astronomers say that the most recent of those cosmic collisions was billions of years later than was thought, making the Milky Way that we know and love a much younger entity than previously believed.

Basically, instead of the stars arriving about eight billion years ago, the new data indicates they may have come from a merger just three billion years ago—much more recently, even in terms of the universe’s total age: 13.77 billion years old.

The astronomers’ finding was made using data from ESA’s Gaia space telescope, which launched in December 2013. Gaia’s third data release was published in 2022, and includes data the researchers say suggests a more recent merger than those previously known. The team’s analysis of the Gaia data was published last month in Monthly Notices of the Royal Astronomical Society.

“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 study co-author Heidi Jo Newberg, an astronomer at Rensselaer Polytechnic Institute, in an ESA release. “New wrinkles of stars form each time the stars swing back and forth through the centre 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 Milky Way's halo of stars as seen by Gaia (left), and how it would look if a merger happened in the ancient past.

The region of interest in the Gaia data is the Milky Way’s inner stellar halo. This area features a stretch with a high concentration of iron and hydrogen. The stars in this stretch have eccentric orbits compared to the surrounding stars. Due to their irregularity, this component of the halo is referred to as the “last major merger,” indicating the most recent intergalactic encounter that helped shape our modern Milky Way. By studying how the Milky Way’s wrinkles smooth out over time, researchers can determine the timing of our galaxy’s mixing with others.

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

The three-billion-year-old collision likely occurred between the Milky Way and a dwarf galaxy, Donlon added. It may be strange to think of the Milky Way as an eclectic agglomeration of stars of different ages, but the recent paper suggests exactly that: We’re just a galactic melting pot, ripe for telescopes to sift through to understand when different ingredients were added.

More: Astronomers Find the Edge of Our Galaxy

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