A Second Jet in Galaxy Mrk 501 Points to a Close Pair of Supermassive Black Holes — Possibly Nearing a Merger

Astronomers studying the Mrk 501 galaxy have identified a second jet of material near its core, an unexpected feature in a system thought to host a single supermassive black hole.

The discovery results from more than two decades of high-resolution radio observations, collected over dozens of sessions and multiple frequencies using a global network of telescopes. The data revealed not only a second jet, but also a change in position over time.

This motion is difficult to explain with a single black hole and is best explained by two supermassive black holes orbiting each other at short distances, each producing its own jet. The results must be published in Monthly Notices of the Royal Astronomical Society.

“We searched for it for so long, and then we were completely surprised that we could not only see a second plane, but even track its movement,” Silke Britzen, head of the search, said in a press release.

How supermassive black holes merge after galactic collisions

Supermassive black holes are thought to be found at the centers of almost all large galaxies, with masses millions or even billions of times greater than those of the Sun. How they can grow to such a size remains an open question, as feeding on surrounding gas alone is unlikely to explain 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.

This final phase, however, was difficult to observe. Theoretical models still struggle to describe it, and clear examples of close pairs in fast orbit have remained rare.

In Mrk 501, a jet has been studied for years because it points toward Earth, making it particularly bright. The second jet has a different angle and only became visible when researchers compared observations taken over time.

Over time, the second jet does not remain fixed. Instead, it moves over weeks, tracing a repetitive path around the galaxy’s core. Sometimes it appears to loop counterclockwise around the nucleus, which corresponds to orbital motion.

This movement suggests that the jets are linked to objects in orbit.

During an observation in June 2022, the alignment briefly bent light into an Einstein ring – likely formed when one black hole passed in front of the other, distorting the light from the jet behind it.

Learn more: The mysterious little red dots of the universe are young black holes

A fast, short-range orbit

By analyzing trends in the motion and brightness of the jets, the researchers estimate that the two black holes complete one orbit every 121 days. They are separated by about 250 to 540 times the distance between Earth and the Sun, extremely close for objects of this size.

At this separation, the couple should lose energy and curl inward. Based on their exact masses, researchers estimate that they could merge within as little as 100 years (a short time frame for such massive objects).

Because Mrk 501 is so far away, even the most powerful instruments cannot resolve the two black holes separately. Instead, their presence is inferred from indirect signals such as the movement and structure of their jets.

A window into a cosmic endgame

Despite decades of study, astronomers have struggled to detect supermassive black holes at this point, as two objects are locked in a narrow, rapidly shrinking orbit. The Mrk 501 system offers one of the clearest examples to date.

In 2023, pulsar timing arrays detected a background of low-frequency gravitational waves – faint ripples in space-time that are believed to come from many pairs of black holes spiraling toward merger.

Mrk 501 could be a unique case where one of these sources is directly identified. As the two black holes get closer together, they should emit stronger gravitational waves, providing the possibility of tracking a supermassive black hole merger as it unfolds.

Learn more: Two black hole mergers emitted gravitational waves, confirming Einstein’s theory of relativity

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