Two large particle jets in the core of a distant galaxy, observed in a first-of-its-kind radio telescope detection, likely represent two black holes in their final phase before merging, a predicted but previously unobserved type of event.
According to a new paper published in the Monthly Notices of the Royal Astronomical Society, which argues that pairs of jets push particles at near light speed from the heart of the Mrk 501 galaxy, these black holes form a binary system, orbiting each other and drawing ever closer to their final collision, an important event for the growth of supermassive black holes.
Central Black Holes
Astrophysicists generally accept that most large galaxies are anchored by central supermassive black holes. At masses equivalent to millions or billions of Suns, scientists struggle to explain how these enormous objects developed. The best bet so far is galaxy merging collisions, where the participants’ central black holes fuse into one, as the steady accretion of gas would occur far too slowly to explain the observed sizes.
While researchers believe that these collisions must be relatively common on a universal scale, they have never been directly observed, and theoretical models have yet to describe their final phase.
The recent study’s research data include dozens of observations collected over 23 years across multiple radio frequencies. Never before have researchers identified two jets emanating from the center of a single galaxy, suggesting the presence of two black holes orbiting one another on their final approach.
“We searched for it for so long, and then it came as a complete surprise that we could not only see a second jet, but even track its movement,” said lead author Silke Britzen of the Max Planck Institute for Radio Astronomy.
A Second Black Hole Discovered
While a jet emanating from Mrk 501 has long been known to astronomers, the discovery of a second one was surprising. The first jet, oriented toward Earth, appears brilliant from our vantage point, but the second, angled differently, presented a greater challenge and was only discovered recently.
Analyzing the data, the astronomers noted that over a one-week cycle, the second jet moved from behind the larger jet and circled it counterclockwise.
“Evaluating the data felt like being on a ship,” said Britzen. “The entire jet system is in motion. A system of two black holes can explain this: The orbital plane sways.”
During one observation, the jet took the curious form of an Einstein ring, in which the jet’s path was so crooked that it appeared ring-shaped. This neatly aligns with a binary black hole system, with the leading black hole’s gravitational lensing shaping the light from the trailing one.
Binary Black Hole Collision Imminent
Analysis of the progression and repeating patterns in jet brightness indicates that binary black holes orbit one another roughly every 121 days. While the objects are between 250 and 540 times as far apart as the Earth and the Sun, considering that they are between 100 million and one billion solar masses each, this is remarkably close on their scale. In fact, if the objects are close to a billion solar masses, their final merger may be only a century away.
The distance of Mrk 501 from Earth is too great for even our most powerful instruments to image, meaning that the black holes’ approach to one another can only be observed indirectly. However, pulsar timing arrays should detect very low-frequency gravitational waves as emitted by the objects as they draw closer together.
Scientists believe that such supermassive black holes produce the gravitational wave background, an idea that was bolstered by 2023 observations from the European Pulsar Timing Array. The team says that Mrk 501 is a perfect target for future research investigating the connection between gravitational wave emission and supermassive black holes.
“If gravitational waves are detected,” explained co-author Héctor Olivares, “we may even see their frequency steadily rise as the two giants spiral toward collision, offering a rare chance to watch a supermassive black hole merger unfold.”
The paper, “Detection of a Second Jet within the Nuclear Core of Mrk 501,” appeared in Monthly Notices of the Royal Astronomical Society on March 27, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.