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The Milky Way Ate One of Its Neighbors 10 Billion Years Ago

Star data shows we gobbled up a galaxy called Gaia-Enceladus about 1/4 the size of the Milky Way, leaving behind telltale signs of the merger

The Milky Way ( NASA/JPL-Caltech/R. Hurt (SSC/Caltech) )
smithsonian.com

New research suggests about 10 billion years ago the Milky Way consumed a smaller galaxy, and the remnants of that cosmic lunch are still swirling around in the Milky Way’s belly.

The long-ago feast was discovered when researchers looked at data collected by the European Space Agency's Gaia space telescope, analyzing data on tens of thousands of stars within 33,000 light years of our own sun, reports Lisa Grossman at ScienceNews. What the data shows is that a group of about 30,000 of those stars aren’t rotating around the galactic center like they should. Instead, they appear to be moving the opposite way.

“That was the first hint,” astronomer Amina Helmi of the University of Groningen in the Netherlands tells Grossman. “When stars move the opposite way, that already tells you that they basically didn’t form in the same place as the majority of the stars in our galaxy.”

Using the Apache Point Observatory Galactic Evolution Experiment in New Mexico, Amina and her colleagues performed a follow-up, looking at the elements making up the stars. The chemical composition showed that the backward moving stars do not contain the same heavy elements as stars like our own. Instead, they appear to be much older, forming before the cycle of birth and death of massive stars spread heavier elements across the universe, according to a new study published in the journal Nature.

The team knew the stars had formed elsewhere, but they didn’t know how they ended up floating around the Milky Way. Using computer simulations, they worked backward, determining that about 10 billion years ago an old galaxy about 25 percent the size of our own was orbiting the Milky Way. Eventually, the two collided. Our cosmic home then slurped up the smaller galaxy, now dubbed Gaia-Enceladus, taking on about 600 million solar masses worth of matter.

Gaia-Enceladus isn’t the only galaxy the Milky Way has gobbled up, Columbia University Astronomer Kathryn Johnston tells Meghan Bartels at Space.com.

“The Milky Way is a cannibal. It has eaten many dwarf galaxies in the past, and we’ve just found a major one that it ate in the past,” she says. “This is like a police investigation — this one in particular, because it’s not a galaxy that we can see today. It’s a dead galaxy, so that makes it kind of fun.”

Research earlier this year shows that the Milky Way is still a little hungry. There are two small gaseous galaxies that orbit our own known as the Large and Small Magellanic Clouds, which also orbit each other. The Milky Way, researchers determined, is currently sipping gas from the Small Magellanic Cloud, using the material to produce new stars and planets.

And the Milky Way isn’t the only cannibal in the area. Our galaxy is part of a squad of about 30 nearby galaxies called The Local Group. As the biggest galaxies of the crew, the Milky Way and the similarly sized Andromeda are the most dominant. Billions of years ago the Milky Way had another neighbor, called M32p, that was the third largest galaxy in the group. Two billion years ago, new research shows, Andromeda gulped up M32p. This could possibly mean that we may be on Andromeda's menu someday—if we don't get there first, that is.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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