An artist’s impression of two black holes that are about to collide.
New observations and analysis reveal two Goliath black holes just 750 light-years apart and closing, as they circle each other in the aftermath of a galaxy merger.
Astronomers from Flatiron Institute and their colleagues have spotted two ghostly Goliaths en route to a cataclysmic meeting. The newfound pair of supermassive black holes are the closest to colliding ever seen, the astronomers announced on January 9 at an American Astronomical Society meeting in Seattle and in a paper published in The Astrophysical Journal Letters.
While close together in cosmological terms at just 750 light-years apart, the supermassive black holes won’t merge for a few hundred million years. In the meantime, the astronomers’ discovery provides a better estimate of how many supermassive black holes are also nearing collision in the universe.
This artist’s conception shows a late-stage galaxy merger and its two newly-discovered central black holes. The binary black holes are the closest together ever observed in multiple wavelengths. Credit: ALMA (ESO/NAOJ/NRAO); M. Weiss (NRAO/AUI/NSF)
That improved head count will aid scientists in listening for the universe-wide chorus of intense ripples in space-time known as
The short distance between the newly discovered black holes “is fairly close to the limit of what we can detect, which is why this is so exciting,” says study co-author Chiara Mingarelli, an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City.
Due to the small separation between the black holes, the astronomers could only differentiate between the two objects by combining many observations from seven telescopes, including 
Telescope observations of two newly discovered supermassive black holes on a collision course. Their host galaxy, left, is a mash-up of two galaxies that have collided. The pink box shows the location of the supermassive black holes. Close observation of the pair, right, reveals two distinct black holes (white spots) only 750 light-years apart. Credit: M.J. Koss et al.
The astronomers found the pair quickly once they started looking, which means that close-together supermassive black holes “are probably more common than we think, given that we found these two and we didn’t have to look very far to find them,” Mingarelli says.
The newly identified supermassive black holes inhabit a mash-up of two galaxies that collided around 480 million light-years away from Earth. Gargantuan black holes live in the heart of most galaxies, growing bigger by gobbling up surrounding gas, dust, stars, and even other black holes. The two supermassive black holes identified in this study are true heavyweights: They clock in at 200 million and 125 million times the mass of our sun.
The black holes met as their host galaxies smashed into each other. Eventually they will begin circling each other, with the orbit tightening as gas and stars pass between the two black holes and steal orbital energy. Ultimately the black holes will start producing gravitational waves far stronger than any that have previously been detected, before crashing into each other to form one jumbo-size
