Astronomers may have spotted the ghost galaxy that hit the Milky Way long ago

The Milky Way survived a galactic hit and run millions of years ago — and astronomers may have finally found the culprit. 

Ten years ago, astrophysicists Sukanya Chakrabarti and Leo Blitz of the University of California, Berkeley, suggested that ripples in the outer gas disk of the Milky Way were caused by a collision with a dwarf galaxy that shook the Milky Way’s gas like a pebble dropped in a pond. The pair made predictions for how massive and distant the galaxy had to be, as well as roughly where it should be found. But none of the known dwarf galaxies that orbit the Milky Way fit the bill (SN: 4/4/15, p. 6). 

Now, Chakrabarti thinks she’s found her quarry, she reported June 12 in St. Louis at the American Astronomical Society meeting and in a study posted on arXiv.org. 

Last year, astronomers using data from the Gaia space telescope discovered a new dwarf galaxy called Antlia 2, which has so few visible stars that its discoverers called it a hidden giant (SN Online: 5/9/18). Antlia 2’s location is “stupidly close” to where Chakrabarti, now at the Rochester Institute of Technology in N.Y., and Blitz predicted that the offending dwarf galaxy should be today, she says. 

Its mass is also close to what the surviving remnant of the colliding galaxy’s mass would be, she estimates. And the collision could even explain why Antlia 2 has so few stars — the encounter with the Milky Way could have stripped many of them away.

To make sure Antlia 2 is the culprit, Chakrabarti and her colleagues have predicted where its stars should be in the next set of Gaia data, due out in 2020 or 2021. “If this is what’s observed a year from now, I’d say it’s indisputable really that Antlia 2 is the dwarf galaxy that we predicted,” she says.