New findings from the Gaia space telescope suggest that the Milky Way may have recently cannibalized a small galaxy, cosmically speaking. In fact, the last major collision between our galaxy and another appears to have occurred billions years later than originally anticipated.
The Milky Way has long been known to have grown through a series of violent collisions in which smaller galaxies are torn apart by the enormous gravitational pull of our solar system’s spiral home. These collisions scatter stars from the engulfing galaxy across the halo that surrounds the main disk of the Milky Way and its characteristic spiral arms. These bouts of galactic cannibalism also send “wrinkles” across the Milky Way, affecting different “families” of stars with different origins and in different ways.
With its ability to pinpoint the position and motion of more than 100,000 stars in the Solar System within a full catalog of stellar bodies on monitors, Gaia aims to retell the history of the Milky Way by counting its wrinkles.
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“As we age, our wrinkles diminish, but our work reveals that the opposite is true for the Milky Way. It’s a kind of cosmic Benjamin Button that shrinks over time,” Thomas Donlon, lead study team at Rensselaer Polytechnic Institute and University of Alabama, said in a statement. in the 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 that it happened billions of years later than we thought.”
These galactic wrinkles were only discovered by Gaia in 2018; this is the first time they have been thoroughly examined to reveal the timing of the collision that created them.
Halo stars move in strange ways
Our galaxy’s halo is populated by stars that have strange orbits, many of which are thought to be “remnants” of galaxies that were once swallowed up by the Milky Way.
Many of these stars are thought to be remnants of the so-called “last big merger,” which refers to the last time the Milky Way experienced a significant collision with another galaxy. Scientists believe that this last 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 filled the Milky Way with stars in orbits that brought them close to the galactic center. The GSE event is thought to have occurred between eight and 11 billion years ago, when the Milky Way was in its infancy.
As of 2020, Thomas and his team have been comparing the Milky Way’s wrinkles to simulations of how galactic collisions and mergers might have created them. However, Gaia observations of these oddly orbiting stars – released as part of the space telescope’s Data Release 3 in 2022 – suggest that these strange stellar bodies may have been deposited by another merger event.
“Using these simulated mergers, we can see how the shapes and number of wrinkles change over time. This allows us to determine the exact time when the simulation best matches what we see today in the actual Gaia data of the Milky Way—the method we used in this one.” ” also a new study,” Donlon explained. “With this, we found that the wrinkles were probably caused by the collision of a dwarf galaxy with the Milky Way about 2.7 billion years ago. We called this event the Virgo Radial Merger.”
“For the stars’ wrinkles to be as bright as they appear in the Gaia data, they must have joined us less than three billion years ago—at least five billion years later than previously thought,” team member Heidi Jo Newberg, also of Rensselaer. Polytechnic Institute, he said. “New star wrinkles form every time the stars swing back and forth through the center of the Milky Way. If they joined us eight billion years ago, there would be so many wrinkles right next to each other that we wouldn’t be able to see.” as a separate function.”
A recent examination of Gaia observations questions 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 region. It also casts doubt on all the stars previously linked to the GSE merger.
“This result – that a large part of the Milky Way has only joined us in the last few billion years – is a big change from what astronomers had previously thought,” said Donlon. “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 believes that the radial merger in Virgo brought a family of other small dwarf galaxies and star clusters into our galaxy, all of which would also have been absorbed by the Milky Way around the same time.
Future investigations and data from Gaia could show whether any objects previously associated with the GSE event are indeed associated with the more recent Virgo Radial Merger.
This new research is the latest in a treasure trove of results based on Gaia data that are rewriting the history of the Milky Way.
Such cosmic revisionism was made possible by Gaia’s unique ability to survey the vast array of stars above Earth, allowing the space telescope to compile an unparalleled map of the positions, distances and motions of some 1.5 billion stars so far.
“The history of the Milky Way 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 since a decade ago, and I think our understanding of these mergers will continue to change rapidly.”
The team’s research was published in May the journal Monthly Notices of the Royal Astronomical Society.