Our galaxy, the Milky Way, is on a collision course. Some four billion years from now, the Milky Way and its large neighboring galaxy, Andromeda, will begin a spiraling gravitational dance, merging over hundreds of millions of years to form one larger object.
Such galaxy mergers are happening all the time, all over the universe, and they are important building blocks for larger-scale cosmic structure. Astrophysicists have many questions about this extraordinary process, but one mystery has proved especially captivating: When large galaxies merge, what happens to the supermassive black holes that decades of observations have revealed to be lurking at their centers?
Logically, these giant black holes—each millions to billions of times heavier than our sun—must collide and merge, too. Such mergers can channel huge volumes of material into the black holes, sparking violent astrophysical outbursts that shape star formation and other processes in their host galaxies. But astronomers have so far only seen snapshots of this long process, from when the black holes are still tens to hundreds of light-years apart. The closer the merging black holes get, the harder they become to distinguish from each other, blurring the picture for theorists seeking to understand how this process works.
Now an international team of scientists has announced the discovery of two active supermassive black holes close to Earth in a new study in the Astrophysical Journal Letters. At an estimated 125 million and 200 million times the mass of the sun, respectively, these black holes sit about 500 million light-years away from us, gobbling up gas and dust at the center of UGC 4211, a galaxy that is still reeling from a merger.
“This pair was really exciting because they’re so close to each other and they’re so nearby,” says Chiara Mingarelli, one of the study’s authors and an astrophysicist at the Flatiron Institute in New York City and the University of Connecticut. Separated by only some 750 light-years, or 230 parsecs, they are the closest pair of black holes that scientists have been able to confirm by measuring multiple wavelengths of light. These black holes’ proximity to Earth and each other may provide a unique opportunity for fundamental studies of giant black hole mergers, as well as one of their most elusive by-products: ripples in spacetime called gravitational waves.
The project began nearly 10 years ago, when astrophysicist Michael Koss started using some of the world’s largest telescopes to search the sky for “active” pairs of supermassive black holes—that is, those that are feeding and burping out blasts of intense radiation. After examining nearly 100 objects, he found six that were actually hidden merging pairs. Of these, UGC 4211’s stood out as being much closer together than the rest.
“You can see it. I mean, literally, you can see on the [images] there are two sources,” says Koss, who works at Eureka Scientific, an astrophysics research institute in California. The high resolution was possible, in part, because of UGC 4211’s proximity to Earth.
This paper “has really pushed the limit” of what is possible to observe, says Zoltan Haiman, an astronomer at Columbia University, who was not involved in the study. Astronomers have observed one pair of binary black holes in even closer proximity to each other before but only with radio telescopes. In the new study, the team confirmed its findings using multiwavelength data from several different telescopes—an important step because the field has had false positives in the past, Mingarelli says. “You can only trick the eye in so many wavelengths,” she says. The team studied the system using the W. M. Keck Observatory in Hawaii, the Very Large Telescope and the Atacama Large Millimeter/submillimeter Array in Chile, and the Hubble Space Telescope in orbit around Earth.
UGC 4211’s two black holes are thought to be midway through their merger. As their extended cosmic duet progresses, the pair will draw even closer together as surrounding swarms of stars and gas siphon away their orbital momentum. The authors predict that the dance will end in approximately 200 million years, when the two supermassive black holes at last fully merge to become one.
“This seems to be right smack in line with a lot of our paradigms, so that’s a good thing. It’s not breaking astrophysics,” says Jeremy Schnittman, an astrophysicist at NASA’s Goddard Space Flight Center, who was not involved in the recent study.
At least, that’s the case for now. Much remains unknown about the mechanics of merging monstrous black holes, especially during later stages, when the black holes approach each other so closely that they cannot be clearly distinguished. “What happens once they’re even closer than this, down at parsecs or less, is still a really big question,” says Sarah Burke-Spolaor, an astronomer at West Virginia University, who was not involved in the new results.
Like many of her peers, Burke-Spolaor is especially fascinated by each merger’s final phase, when the black holes spiral together so violently that they shake the fabric of spacetime itself, producing copious gravitational waves. After first detecting such emissions in 2015—a discovery that netted a Nobel Prize in Physics—astronomers now routinely study these last gasps from merging black holes using specialized observatories that are as different from light-gathering telescopes as ears are from eyes. Most of those studies, however, concern pairs of black holes that are far smaller than their supermassive kin. Tuning in to the immense gravitational waves from the universe’s largest merging black holes requires a new generation of even more advanced observatories that scientists and engineers have scarcely begun to build.
The payoff should be worthwhile: Such huge mergers are thought to be the most common contributor to the “gravitational wave background,” the as-yet-undetected totality of spacetime ripples from sources scattered across the entire observable universe, imprinted across the entire sky. Detecting and mapping this background, Mingarelli says, would yield “the cosmic merger history of supermassive black holes” and a wealth of other cosmologically vital information. And studying the fine details of merging systems like UGC 4211 can help researchers better understand what they’re seeing if or when they finally manage to glimpse the universe’s gravitational wave background.
Koss hopes that future studies of UGC 4211 will be especially fruitful, given its proximity to Earth. This system is easier to observe from our place in the universe, “just like you can see the leaves on the nearby trees in the forest, not distant ones,” he says.
The discovery also suggests that there may be more merging black holes to observe than we previously thought because Koss found it from a sample of fewer than 100 active black holes. “There’s always a chance that the study just got lucky,” Burke-Spolaor says. But even if it was a fluke, scientists will continue searching for more examples to fill in the final steps of this cosmic choreography.