Astronomers studying the galaxy Mrk 501 have identified a second jet of matter near its core, an unexpected feature in a system thought to host a single supermassive black hole.
The discovery comes from more than two decades of high-resolution radio observations, collected across dozens of sessions and multiple frequencies using a global network of telescopes. The data revealed not only a second jet, but that it shifts position over time.
That motion is difficult to explain with a single black hole and is best explained by two supermassive black holes orbiting each other at close range, each producing its own jet. The findings are to be published in Monthly Notices of the Royal Astronomical Society.
“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 Silke Britzen, research lead, in a press release.
How Supermassive Black Holes Merge After Galaxy Collisions
Supermassive black holes are thought to sit at the center of nearly every large galaxy, with masses millions to billions of times greater than the sun. How they grow so large remains an open question, as feeding on surrounding gas alone is unlikely to account for their size, suggesting that mergers play a major role.
These mergers follow galaxy collisions, which are common. As galaxies combine, their central black holes move closer together, forming a pair that later merges into one.
That final phase, however, has been hard to observe. Theoretical models still struggle to describe it, and clear examples of close, rapidly orbiting pairs have remained rare.
In Mrk 501, one jet has been studied for years because it points toward Earth, making it appear especially bright. The second jet is angled differently and only became visible when researchers compared observations taken over time.
Over time, the second jet does not remain fixed. Instead, it shifts over weeks, tracing a repeating path around the galaxy’s core. At times, it appears to loop anticlockwise around the core, consistent with orbital motion.
That motion hints at the jets being tied to objects in orbit.
During one observation in June 2022, the alignment briefly bent the light into an Einstein ring — likely formed as one black hole passed in front of the other, distorting the light from the jet behind it.
Read More: The Universe’s Mysterious Little Red Dots Are Young Black Holes
A Rapid Orbit at Close Range
By analyzing patterns in the jets’ motion and brightness, the researchers estimate that the two black holes complete an orbit every 121 days. They are separated by roughly 250 to 540 times the distance between Earth and the sun, extremely close for objects of this size.
At that separation, the pair is expected to lose energy and spiral inward. Depending on their exact masses, the researchers estimate they could merge in as little as 100 years (a short timescale for such massive objects).
Because Mrk 501 is so far, even the most powerful instruments cannot resolve the two black holes separately. Instead, their presence is inferred from indirect signals such as the motion and structure of their jets.
A Window Into a Cosmic Endgame
Despite decades of study, astronomers have struggled to catch supermassive black holes at this stage, when two objects are locked in a tight, rapidly shrinking orbit. The system in Mrk 501 offers one of the clearest examples yet.
In 2023, pulsar timing arrays detected a low-frequency gravitational wave background — faint ripples in spacetime thought to come from many black hole pairs spiraling toward merger.
Mrk 501 could be a unique case where one of those sources is identified directly. As the two black holes draw closer, they should emit stronger gravitational waves, offering a chance to follow a supermassive black hole merger as it unfolds.
Read More: Two Black Hole Mergers Emitted Gravitational Waves, Upholding Einstein's Theory of Relativity
Article Sources
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- This article references information from an upcoming study that will be published in Monthly Notices of the Royal Astronomical Society: Detection of a second jet within the nuclear core of Mrk 501
