The universe is a large place. Really, really large. As a result of its sheer scope, astronomers and scientists are still unable to figure out tons of its mysteries, including the Milky Way once having a sibling.
Two billion years ago, as it turns out, the Milky Way had a sibling galaxy called M32p. Then, Andromeda just ate it.
Andromeda, the Milky Way's nearest and largest galactic neighbor, essentially gobbled up M32p, according to a new study published in Nature Astronomy. Think of galaxies as hungry monsters who need to feed on other monsters to keep growing — Andromeda, in particular, had a voracious appetite: apart from M32p, Andromeda is believed to have eaten hundreds more of smaller galaxies around it.
That's quite unfortunate because when larger galaxies eat up smaller ones, it's downright near impossible to trace the smaller galaxies individually. M32p isn't small, though. In fact, it would have been the third largest galaxy in the Local Group had Andromeda not eaten it. It was at least 20 times larger than any galaxy the Milky Way has ever merged with.
The Milky Way's Long Lost Sibling
For a long time, astronomers have known about Milky Way's long lost sibling — that's where the halo of stars around Andromeda comes from. It's extremely difficult to determine exactly how many galaxies it has eaten up. So researchers looked at the evidence and created computer simulations, which then helped them reconstruct the events that lead to M32p's death billions of years ago.
The said halo is what originally caught the attention of researcher Richard D'Souza and professor Eric Bell, both from the astronomy department at the University of Michigan. New computer simulations revealed that the outer halo around Andromeda was the result of one large galaxy having been violently torn apart.
"Astronomers have been studying the Local Group — the Milky Way, Andromeda and their companions — for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it," said Bell.
What About M32?
The revelation also gave birth to an entirely new mystery: Andromeda has a small satellite galaxy called M32, but scientists don't know where it came from. Using this theory, they can extrapolate that the dense, compact galaxy might actually be the remnants of M32p.
The discovery is astounding because it reveals new theories about the universe scientists would have never been able to come up on their own. That's largely thanks to computer simulations, of course, which help astronomers get a better sense of how galaxies evolve over time. It also shows just how terrifying Andromeda is.
It's not going to eat the Milky Way anytime soon, but that's going to happen eventually.