The Andromeda Galaxy Ate The Milky Way’s Lost Sibling

New simulations show Andromeda absorbed the large galaxy M32p about 2 billion years ago

Andromeda NASA/JPL/California Institute of Technology

Once upon a time, there were three huge galaxies in the Local Group that were the best of friends, Andromeda, The Milky Way and the Milky Way’s slightly smaller sibling, M32p. For a long time the three discs swirled away near each other, sucking up matter and other smaller galaxies. But one day, Andromeda got so hungry that she crashed into M32p, gobbling her up and ripping her to shreds, leaving a trail of cosmic guts behind. Hannah Devlin at The Guardian reports that scientists have just discovered this story of galactic murder by studying Andromeda’s halo, and the tale will help them refine the science behind galaxy formation and the fate of our own galaxy.

Like our own Milky Way, Andromeda is a spiral galaxy. But it has some unique features, including a faint halo of stars orbiting it and a small but very dense satellite galaxy called M32. Researchers believed that this starry halo was created as Andromeda absorbed hundreds of smaller galaxies over time, leaving behind galactic crumbs. But according to a press release, astronomers using computer models of galaxy formation found that the best explanation for Andromeda’s halo was not the gradual gobbling of small galaxies but one giant galactic meal. And that meant those remnant stars could be used to reconstruct the galaxy Andromeda ate.

“It was a ‘eureka’ moment,” says University of Michigan astronomer Richard D’Souza, lead author of the paper in Nature Astronomy. “We realized we could use this information of Andromeda’s outer stellar halo to infer the properties of the largest of these shredded galaxies.”

“It’s kind of like a child eating dinner, and then looking on the floor afterwards and finding breadcrumbs all around,” D’Souza tells Devlin. “You know what’s been eaten.”

Ryan F. Mandelbaum at Gizmodo reports that the model explains a number of features of Andromeda. The stars in Andromeda's halo are all roughly 2 billion years old, as are about one fifth of the stars in Andromeda proper, suggesting a massive galactic collision and burst of star-formation activity in the past. The stars in the halo are also composed of elements heavier than hydrogen and helium, as does the super-compact galaxy M32, suggesting that it was once the core of a galaxy absorbed by Andromeda, spit out like a peach pit.

That means the halo and core are all that remain of M32p, which would have been the third largest galaxy in our Local Group, a family of about 50 galaxies in a region of space about 10 light-years across. The galaxy would have been a massive lunch, about 20 times larger than anything our own Milky Way has ever eaten.

“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,” co-author University of Michigan astronomer Eric Bell says in the release.

The find has already upended some of what we know about galaxy formation. For instance, according to the release it was believed that the merging of two large galaxies would destroy any spiral galaxy and lead to the formation of an elliptical galaxy, an amorphous blob-like galaxy. But Andromeda and its spiral survived. A study from earlier this year also indicated that it undwent a merger 1.8 to 3 billion years ago, but instead of bloating into an elliptical galaxy as it ate M32p, that study shows that its disk thickened and the galaxy underwent a massive round of star formation.

Mandelbaum reports that other astronomers say the simulation makes sense, though, of course the model is just the best guess as to what happened. “Although I find the evidence that they have collected in this paper very convincing, it would be worth having detailed simulations that try to follow up on this model to validate it,” Monica Valluri, a University of Michigan astronomy professor, says.

Devlin reports that the finding also reveals something about our own eventual fate. The Milky Way and Andromeda are hurtling toward each other at 248,000 miles per hour and will eventually collide in about four billion years, creating a new galaxy some have dubbed Milkomeda. “We will be shredded and be part of the galactic halo,” D’Souza says. Despite the massive collision, the chances of it impacting Earth or our solar system directly are minuscule, explains physicist Dave Goldberg at i09, since so much of space, is, well, empty space. But the worry is misplaced anyway. By the time of impact, our sun will have become a red giant, expanding so much that it will have engulfed our planet, cooking whatever desendents of humanity still remain.

Though we will be gone, there's still a chance our beloved galaxy will be more than just a snack; there is some hope that Andromeda won’t simply absorb us as it did M32p. Previously astronomers believed Andromeda was 2 or 3 times the size of the Milky Way, but recent research suggests that the two galaxies are about the same size, meaning we’ll have a fighting chance at coming out on top when our spirals finally tangle.

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