13.04.2026

Theorists can simulate the merger of a supermassive black hole binary (an example shown above). Finding one has proved harder.IMAGE: NASA GODDARD SPACE FLIGHT CENTER/SCOTT NOBLE; SIMULATION DATA: D'ASCOLI ET AL. 2018

Astronomers routinely see galaxies crashing into each other and combining. But the final phase of these cosmic mergers has long proved elusive: two supermassive black holes, each once occupying the center of its own galaxy, closely circling each other within a single, combined galaxy. Now, researchers say they have found compelling evidence of such a pairing. A distant galaxy seems to be firing off two beams of radiation from its center at different angles—a sign that a pair of supermassive black holes lurks at its heart.

The two behemoths—each with a mass as large as 1 billion Suns—seem to orbit each other every 121 days. The duo’s dance cannot last forever. In as little as 100 years, the researchers say, the black holes should collide, shaking spacetime itself in a titanic burst of gravitational waves. That final burst “would be a really fantastic gravitational wave signal,” says team leader Silke Britzen of the Max Planck Institute for Radio Astronomy.

Previous binary black hole candidates have failed to be confirmed, so independent experts are cautious about the discovery. “It’s messy, but it does look like two jets, which would require two black holes,” says Zoltan Haiman, an astrophysicist at Columbia University. “To be honest, it’s very complicated. … I would say it’s still only a candidate.”

Galaxies can be seen across the cosmos merging with each other. Astronomers know our own Milky Way has consumed smaller neighbors, based on filamentous “streams” of starsand chemical differences between stellar populations, and they forecast it may collide with the nearby Andromeda galaxy in 4.5 billion years. The black holes thought to lie at the heart of nearly all galaxies should merge, too, which helps explain how some reach masses many billion times that of the Sun.

During a galaxy merger, the black holes are unlikely to collide head-on. Instead, they shoot past each other and settle into orbit around each other, forming a binary pair. Over hundreds of millions of years, the two black holes inch closer to each other, losing energy initially via friction with the galaxy’s stars and gas and later through the emission of gravitational waves, ultimately spiraling into a merger.

Astronomers have long searched for evidence of supermassive black hole binaries within a single galaxy, but even the sharpest eyed telescopes can’t resolve two black holes when they’re right next to each other. Instead, astronomers look for periodic signals coming from galactic centers that would indicate orbiting black hole pairs. But galaxy cores are messy places, often full of spinning disks of gas and jets of material emanating from the black hole’s poles. It has proved hard to pick out the definitive signal of binary orbits.

Now, Britzen’s team believes it has found such a signal, as described in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society. The candidate signal comes from Markarian 501, a galaxy some 500 million light-years from Earth whose nucleus is so active, it’s referred to as a “blazar.”

The center of Markarian 501 is rapidly consuming matter while also channeling some of it into a jet of high energy particles and powerful radio waves that just happens to be pointing roughly toward Earth. Oddities in that radio signal led some to suspect there might be a black hole binary at the heart of Markarian 501, so Britzen and her team took a look.

If the black hole producing the jet was one of a pair, the jet would appear to move over time. So Britzen’s team downloaded 23 years’ worth of data on the blazar gathered by the Very Long Baseline Array, a network of radio dishes spanning the United States. At first, the researchers didn’t see any motion, so they shifted to a higher frequency to track what was happening closer to the black hole. Suddenly they saw a different signal—one pointed away from Earth. “That was not to be expected,” Britzen says.

The team concluded that the second signal represented a second jet, presumably coming from a second black hole. They also noticed variations in the brightness of the source that repeated every 121 days, suggesting the orbital period of the two black holes. “It is intriguing that this [second jet] seems to behave differently than the other jet … suggesting a different origin,” says Daniel D’Orazio of the Space Telescope Science Institute. But, he says, finding a binary so close to merging would be almost too good to be true. “If this is confirmed as a binary then we are either very lucky, or the demographics [are] not as we expected, implying many more such systems,” he says. If it were the latter, he asks, why haven’t observers found more evidence of these binaries by now?

Britzen believes she won’t have to wait that long to see whether she’s right. If the binary is that close to merging, astronomers should be able to measure its orbital period shorten over the next decade. In addition, the pair should already be giving off gravitational waves that may be detectable by measuring slight changes in the metronomic signals of networks of fast-rotating stars called pulsars. “There is hope,” Britzen says.

Quelle: AAAS