Andromeda galaxy likely swallowed Milky Way's sibling

Researchers discovered evidence that Andromeda likely collided with the Milky Way's sister galaxy billions of years ago.
By Joseph Scalise | Jul 24, 2018
Scientists from the University of Michigan have found evidence that the nearby Andromeda galaxy clashed with and devoured the Milky Way's long-lost sibling M32p, a new study in Nature Astronomyreports.

The Milky Way and Andromeda are linked because they are two of the largest galaxies in our small section of the universe. However, in the new study astronomers found that Andromeda once devoured the third largest member of the family roughly 2 billion years ago.

"Astronomers have been studying the Local Group the Milky Way, Andromeda and their companions for so long," said study co-author Eric Bell, a professor of astronomy at the University of Michigan, according to Space.com. "It was shocking to realize that the Milky Way had a large sibling, and we never knew about it."

The team made this new discovery by using computer simulations to reveal that almost all of the stars in the outer reaches ofAndromeda's "halo" -- the roughly spherical region surrounding the galaxy's disk -- came from a single event.

That is important because scientists used that information to properly infer theproperties of the largest of those long-dead galaxies.

That process, combined with recently created models, enabled the team to properly date the merger to roughly 2 billion years ago. In addition, they also managed to reconstruct some details from the long-dead galaxy.

That showed M32p was roughly 20 times the size of any galaxy the Milky Way has ever merged with. In addition, scientists believe that Andromeda's satellite galaxy M32 -- which is one of the most compact galaxies in the universe -- is likely made up of the remains from M32p.

The timing of the ancient merger is important because it reinforces what scientists understand about galaxy formation. It also matches up with previous research that Andromeda merged with another large galaxy between 1.8 and 3 billion years ago.

In that way, the finding may help scientists better understand the mechanisms behind galaxy mergers and shed light on the way they evolved over time.

"The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago," added Bell, according toPhys.org.

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