Scientists map Andromeda's history of eating nearby galaxies

Oct. 3 (UPI) — Galaxies grow by consuming other galaxies. They’re cannibalistic. But according to a new study, the Milky Way’s closest large neighbor, the Andromeda Galaxy, has a particularly voracious appetite.

Scientists knew Andromeda had swallowed up several smaller galaxies, but the latest survey offered new details about the galaxy’s history — a history of violence that data suggests stretches back at least 10 billion years.

And according to the new research, published this week in the journal Nature, the Milky Way could be Andromeda’s next meal.

“The Milky Way is on a collision course with Andromeda in about four billion years. So knowing what kind of a monster our galaxy is up against is useful in finding out the Milky Way’s ultimate fate,” Dougal Mackey, professor at Australia National University’s Research School of Astronomy and Astrophysics, said in a news release. “Andromeda has a much bigger and more complex stellar halo than the Milky Way, which indicates that it has cannibalized many more galaxies, possibly larger ones.”

To trace Andromeda’s dietary history, scientists analyzed the dense groups of stars, or globular clusters, that orbit the massive galaxy.

“By tracing the faint remains of these smaller galaxies with embedded star clusters, we’ve been able to recreate the way Andromeda drew them in and ultimately enveloped them at the different times,” Mackey said.

According to the latest research, Andromeda’s clusters suggest the galaxy’s history is defined by two bouts of binge eating. But mysteriously, the meals arrived from two different directions.

“This is very weird and suggests that the extragalactic meals are fed from what’s known as the ‘cosmic web’ of matter that threads the universe,” said Geraint Lewis, professor at the University of Sydney’s School of Physics.

Scientists determined the feedings also occurred on the “plane of satellites,” the plane on which the majority of the dwarf galaxies orbiting Andromeda lie. Previous studies showed such planes are fragile and likely to be destroyed by Andromeda’s gravity within a few billion years.

“This deepens the mystery as the plane must be young, but it appears to be aligned with ancient feeding of dwarf galaxies. Maybe this is because of the cosmic web, but really, this is only speculation,” Lewis said. “We’re going to have to think quite hard to unravel what this is telling us.”

Lewis and Mackey call their work cosmic archaeology. According to the researchers, by unraveling Andromeda’s history, they can gain new insights into how large galaxies grow and evolve — insights that can help cosmologists better understand the Milky Way’s past, present and future.

“One of our main motivations in studying astronomy is to understand our place in the Universe. A way of learning about our galaxy is to study others that are similar to it, and try to understand how these systems formed and evolved,” Mackey said. “Sometimes this can actually be easier than looking at the Milky Way, because we live inside it and that can make certain types of observations quite difficult.”

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