Researchers at the University of Michigan say they've discovered that the Milky Way had a large "sibling" galaxy that was shredded by another galaxy 2 billion years ago.

Researchers Richard D'Souza, a postdoctoral researcher, and Eric Bell, a professor of astronomy, used consumer simulations to determine that the Milky Way's nearest galactic neighbor, the Andromeda galaxy, consumed a third galaxy known as M32p.

Their findings, released Monday and published in Nature Astronomy, will help astronomers understand how the Milky Way and other "disk" galaxies evolve and survive large mergers.

"Astronomers have been studying the Local Group — the Milky Way, Andromeda and their companions — for so long," Bell said. "It was shocking to realize that the Milky Way had a large sibling, and we never knew about it."

Bell and D'Souza say M32p was about a third to half the size of the Milky Way, which would have made it the third largest galaxy in the Local Group, and that it was probably a spiral galaxy, like the Milky Way.

D'Souza says the common understanding among researchers was that a merger between Andromeda, otherwise called M31, and M32p would have left an elliptical galaxy, destroying Andromeda's disk.

But that's not what happened.

"The disk of Andromeda still survived. The collision probably thickened the disk of M31," D'Souza said. "The lifetime of the universe is about 13.7 billion years, and we propose (the merger) happened 2 billion years ago, so this is a fairly recent merger. That's really surprising."

Bell predicts there will be major interest among researchers in what he calls the "miracle of the survival of the Andromeda disk."

"It's really quite counterintuitive that it made it through such a big interaction," he said. "The astronomical community will be trying to figure out how the disk survived. The disk thickened: how the heck did that happen?"

The research doesn't mean much for the Milky Way at present, but it may have implications for our galaxy 2 billion years down the road, D'Souza says.

"(There is) a small galaxy surrounding the Large Magellanic Cloud. Eventually, 2 billion years later, it could merge with the Milky Way," he said. "Will the Milky Way survive the collision? This merger with Andromeda shows that it probably will survive."

Bell says there is much more scientists will continue to discover about Andromeda.

"What other things happened to Andromeda as a result of this collision?" he said. "There was a big burst of star formation in Andromeda 2 billion years ago. There were other things that might have happened to Andromeda, too."

D'Souza echoed Bell's thoughts, saying this research will provide scientists with new lines of questioning to follow for other galaxies.

"Once you know the merger history of these galaxies, you can ask what their mergers do?" he said. "Do they thicken the disk? Do they stop the formation of stars? You can start answering these questions."


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