Caught: A ghost galaxy that may have hit ours long ago

The star system Antlia 2 turned up not far from where astronomers predicted the culprit should be

Antlia 2, a dim galaxy that orbits the Milky Way, appears at the right in this illustration. It’s a bit bigger but has fewer stars than the Large Magellanic Cloud (left), which also orbits our Milky Way.

V. Belokurov/Univ. of Cambridge/CCA, based on the images by Marcus and Gail Davies and Robert Gendler

The Milky Way survived a galactic hit and run. It took place millions of years ago. But astronomers may have finally found the culprit. Their suspect is a dwarf galaxy called Antlia 2.

That a galaxy likely hit our galaxy is not new. Sukanya Chakrabarti and Leo Blitz suggested it 10 years ago. Back then, both astrophysicists were working at the University of California, Berkeley. They pointed to ripples in the outer edges of the Milky Way as evidence of a collision with some dwarf galaxy. A dwarf galaxy is far smaller than a massive spiral one, like our Milky Way.

The collision probably shook the Milky Way’s gas like a pebble dropped into a pond, the scientists said. They predicted how massive the galaxy that hit the Milky Way had to be. They even estimated roughly where it should be.

Still, astronomers couldn’t find it. None of the small galaxies known to orbit the Milky Way fit the bill.

Then last year, other astronomers discovered Antlia 2. They found this dwarf using the Gaia space telescope. Antlia 2 doesn’t have many visible stars. That’s why the astronomers called it a hidden giant. Intrigued, Chakrabarti looked at the data for this galaxy late last year. And she thought it might just be the missing culprit. Antlia 2’s location, she says, is “stupidly close” to where she and Blitz predicted that the dwarf galaxy should be today.

Antlia 2’s mass also is close to what the surviving remnant of the colliding galaxy’s mass would be, she estimates. Chakrabarti now works at the Rochester Institute of Technology in New York. The collision might even explain why Antlia 2 has so few stars, she says. Its encounter with the Milky Way could have stripped many of them away.

Chakrabarti reported the finding June 12. at the American Astronomical Society meeting, in St. Louis, Mo. Chakrabarti also posted a study about the findings on arXiv.org.

She and her colleagues, however, are not yet positive Antlia 2 is to blame. To make sure, they have predicted where the culprit’s stars should be. They can check it in the next set of Gaia data. Those data are due out in 2020 or 2021.

“If this is what’s observed a year from now,” she says, “I’d say it’s indisputable, really, that Antlia 2 is the dwarf galaxy that we predicted.”

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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