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The Milky Way contains more than 100 billion stars, but it didn’t come from all of them honestly. At least a dozen times in the past 12 billion years, the Milky Way has collided with a nearby galaxy and devoured it, swallowing that neighbor’s stars and mixing them into an ever-increasing stew of stolen suns.
With each galactic merger, the shape, size and motion of our galaxy changed forever, eventually becoming the iconic spiral we recognize ourselves today. Now, in a recent study published in the October 2020 issue of the journal Royal Astronomical Society Monthly Notices, the researchers attempted to carry out that spiral. Using artificial intelligence (AI) to match distinct clusters of stars based on their age, movements and chemical compositions, the team found evidence of five large-scale galactic mergers (each involving 100 million stars or more) dating back to more than 10 billion years. an ancient collision that has never been described before.
Related: 11 fascinating facts about our Milky Way galaxy
This newfound incident with the so-called Kraken galaxy not only helps fill in the Milky Way’s mysterious family tree, but could also help astronomers reconstruct what our galaxy looked like in its early days, the study authors said.
“The collision with Kraken must have been the most significant merger ever experienced by the Milky Way,” study lead author Diederik Kruijssen, astronomer at the University of Heidelberg in Germany, he said in a statement. “The merger with Kraken occurred 11 billion years ago, when the Milky Way was four times less massive [than today]. Consequently, the collision must have really transformed the appearance of the Milky Way at that time. “
In their new study, Kruijssen and his colleagues used computer simulations to analyze everything known globular clusters – old dense spheres of up to 1 million stars that all formed at the same time as each other – within the Milky Way. Our galaxy is home to at least 150 of these clusters, which astronomers believe are “fossils” of the ancient galaxies that the Milky Way devoured during its long and hungry history.
The researchers trained an artificial intelligence algorithm to identify globular clusters based on the shared properties of stars, initially running the algorithm on thousands of simulated galaxies. Once the algorithm was able to accurately predict the formation, evolution and destruction of globular clusters in those fictional galaxies, the team unleashed their AI on the Milky Way.
Using data obtained from the Gaia space probe (which gave us the extension most complete map of the Milky Way), the algorithm analyzed the ages, motions and chemical compositions of known globular clusters in our galaxy in order to recreate the cosmic mergers that brought them there. The team’s analysis accurately predicted four known mergers in the Milky Way’s past, including the so-called Gaia sausage merger, which added several billion stars to the bulge of our galaxy about 9 billion years ago, as well as the previously unknown Kraken merger.
And that fusion was a beast. According to the team’s findings, the Kraken may have been the largest and oldest galactic collision in the history of the Milky Way. The merger occurred when the Milky Way was only a fraction of its current size and may have added 13 globular clusters to our galaxy that are still identifiable today. While the Gaia sausage melt eventually added more solar mass to the Milky Way (more than 20 globular clusters) than the Kraken, our galaxy was considerably larger when the sausage melt occurred and was likely less susceptible to major structural changes, the researchers wrote.
This new merger is just a small piece of the puzzle. As the road to galaxy formation is littered with collisions like these, it is likely that many other small-scale mergers also contributed to the Milky Way we know today. Astronomers suspect that at least 15 other mergers may be lurking in our galaxy’s past, each involving 10 million stars or more, and their remains are just waiting to be found in the globular bowels of our galaxy.
“Debris from more than five progenitor galaxies has now been identified,” Kruijssen said. “With current and future telescopes, it should be possible to find [evidence of] the mall.”
Astronomers have about 3 to 4 billion years to figure this out. Subsequently, another merger will occur that alters the galaxy when the neighboring one Andromeda galaxy (currently 2.5 million light-years away) and the Milky Way will inevitably collide. That’s not always the case: just when you think you know a galaxy, it changes you again.
Originally published on LiveScience.
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