‘Benjamin Button’ effect allows astronomers to better calculate ‘last big crash’
TROY, N.Y. — A stunning new study may upend everything we know about our cosmic home — the Milky Way galaxy. According to researchers from the Rensselaer Polytechnic Institute, our galaxy may have collided with another galaxy billions of years later than scientists previously thought. In fact, according to the study, the last time the Milky Way collided with another star cluster, the Earth had already formed. What a light show that must have been!
The findings, in a nutshell
In their groundbreaking study, published in the Monthly Notices of the Royal Astronomical Society, the team of astronomers uncovered compelling evidence that the Milky Way galaxy experienced a massive merger event with a dwarf galaxy about six billion years later than believed. This discovery challenges the long-held theory that the last major merger, known as the Gaia-Sausage/Enceladus (GSE), occurred an astounding eight to 11 billion years ago. Instead, the new research suggests that the debris we see in the Milky Way’s stellar halo — the diffuse sphere of stars surrounding the galaxy’s disk — is the result of a collision that took place a mere one to two billion years ago, a cosmic blink of an eye in astronomical terms.
Researchers Heidi Jo Newberg and Tom Donlon focused on the “wrinkles” in our galaxy, which form when other galaxies smash into the Milky Way.
“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,” says Donlon, lead author of the new Gaia study, in a media release. “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.”
“For the wrinkles of stars to be as obvious as they appear in Gaia data, they must have joined us no less than three billion years ago – at least five billion years later than was previously thought,” adds Newberg. “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.”
Takeaways
Despite these limitations, the researchers argue that their findings are robust and consistent with other lines of galactic evidence. For instance, the observed stellar shells and substructures in the Milky Way’s halo are better explained by a recent collision rather than an ancient one, as older debris would have had more time to phase-mix and become less pronounced.
Moreover, the study provides a compelling alternative to the GSE scenario, which has faced increasing scrutiny in recent years. Some researchers have argued that the chemical and kinematic signatures attributed to the GSE could be explained by other processes, such as secular evolution or multiple smaller mergers.