The Milky Way once had a massive sibling galaxy that was shredded and cannibalized by the neighboring Andromeda galaxy 2 billion years ago, a study reported Monday.
Even though the Milky Way's sibling, named M32p, was mostly shredded, it left behind a rich trail of evidence: an almost invisible halo of stars larger than the Andromeda galaxy.
M32p was the third-largest member of the Local Group of galaxies, which includes the Milky Way, Andromeda and their companions.
"It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said University of Michigan astronomer Eric Bell, a co-author of the study.
The huge, spiral Andromeda is a prolific cannibal and probably shredded hundreds of its smaller galaxies over time, according to Space.com.
Researchers used computer simulations to figure out that even though many other galaxies were "eaten" by Andromeda, most of the stars in Andromeda's outer faint halo were mainly from shredding a single large galaxy – M32p.
"It was a 'eureka' moment. We realized we could use this information of Andromeda's outer stellar halo to infer the properties of the largest of these shredded galaxies," said the study's lead author, Richard D’Souza, a postdoctoral researcher at the University of Michigan.
Discovering and studying this decimated galaxy will help astronomers understand how disk galaxies like the Milky Way evolve and survive large mergers.
The new research was published in the peer-reviewed British journal Nature Astronomy.