We’ve seen ourselves in the heavens. A giant spiral galaxy 180 million light years from Earth not only resembles the Milky Way but also boasts a pair of interacting galaxies that look like our galaxy’s two brightest satellites.
At least 50 galaxies orbit the Milky Way. Most have run out of gas because they’ve spent more time close enough to our galaxy for it to steal their gas. But two of the nearest satellites – the Large and Small Magellanic Clouds – have been in our neighbourhood for less time, and still possess lots of gas that spawns new stars. The two galaxies are respectively 160,000 and 200,000 light years from us and 75,000 light years from each other.
This arrangement is rare. Most giant galaxies don’t have even one star-making companion nearby, let alone two. That’s probably because a giant galaxy strips small neighbours of gas, thwarting their ability to make new stars.
Sanjaya Paudel and Chandreyee Sengupta at Yonsei University in Seoul, South Korea, looked through images of nearly 20,000 small galaxies for a pair that resides near a giant galaxy. “It’s obviously very difficult,” says Paudel.
Bridge of stars
Nevertheless, the astronomers succeeded, finding a giant barred spiral galaxy in the constellation Hydra named NGC 2718 that resembles the Milky Way. Moreover, it has two bright star-forming companions named UGC 4703. A bridge of young stars connects the two smaller galaxies, indicating they are interacting with each other, just as the Magellanic Clouds are.
“They have definitely found a better analogue than any of the cases we presented,” says Philip James at Liverpool John Moores University, who in 2011 published the results of a search for such systems and found them exceedingly rare.
“This one is particularly interesting because it is clear that the two smaller galaxies are interacting,” says Gurtina Besla at the University of Arizona in Tucson. “That wasn’t as clear in any of the other existing analogues.”
How can star-making galaxies thrive next to a gas-grabbing giant galaxy? The Magellanic Clouds probably fell toward us only recently, so the Milky Way hasn’t had time to steal much of their gas. Paudel says the same explanation may hold for the newfound galactic trio in Hydra.