Galaxy collisions occur frequently throughout the universe, and the black holes at the centers of merging galaxies are expected to eventually coalesce. As two galaxies merge, their central black holes should move closer together, begin orbiting one another, gradually spiral inward and ultimately become a single object.

Despite this prediction, scientists still struggle to model the final stages of supermassive black hole mergers. Even more puzzling, astronomers have found abundant evidence of colliding galaxies but have not reliably identified a pair of supermassive black holes orbiting very closely together.

A second jet may reveal a hidden black hole

An international research team led by Silke Blitzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has found direct observational evidence suggesting that such a pair exists at the center of the galaxy Mrk 501. The research is set to be published in Monthly Notices of the Royal Astronomical Society.

Astronomers have long known that the black hole at the center of Mrk 501 produces powerful jets of particles that travel through space at nearly the speed of light. These jets originate in the extreme environment surrounding a black hole, where intense magnetic fields help accelerate matter outward.

To investigate the system, researchers examined detailed radio observations collected at multiple frequencies over several weeks during approximately 23 years of observations. The extensive data set revealed something unexpected: a second jet next to the familiar one. The team was also able to track its movements.

The observation provides what researchers describe as the first direct imaging evidence for such a system at the center of a galaxy and strongly suggests the presence of a second supermassive black hole. However, the two black holes are still too close together in the sky to be resolved as separate objects.

“We had been looking for it for a long time, and it was a complete surprise to not only see the second jet but even be able to track its movements,” said Silke Blitzen.

Two supermassive black holes appear to be in a cosmic dance

The previously known jet is directed toward Earth, making it appear bright and relatively easy to observe. The newly identified jet points in a different direction, which may explain why it remained hidden for so long.

After only a few weeks of observations, astronomers noticed that the position of the second jet was changing dramatically. It appeared to emerge from behind the larger black hole and sweep counterclockwise around it. The repeated motion suggested that the entire system was in regular orbital motion.

“Evaluating the data was like being on a ship. The entire jet system is in motion. This can be explained by a system of two black holes. The orbital plane is shaking,” Blitzen explained.

Researchers also identified an unusual event in June 2022, when radiation from the system appeared to form a ring. This phenomenon, known as an Einstein ring, occurs when gravity bends light into a ring-like shape.

One possible explanation is that the two black holes and Earth were temporarily aligned. In this configuration, the gravity of the foreground black hole could have acted as a lens, bending light from the second jet behind it. This effect, known as gravitational lensing, provides another potential clue about the system.

Binary black holes could merge in as little as 100 years

By analyzing recurring patterns in the jets’ changes and brightness, the researchers calculated that the two black holes appear to orbit each other approximately once every 121 days.

Their estimated separation is only 250 to 540 times the distance between Earth and the Sun. Although that distance sounds enormous, it is surprisingly small for objects with a combined mass estimated at between 100 million and 1 billion times the mass of the Sun.

Such a tight orbit could have dramatic consequences. Depending on the actual masses of the black holes, they could continue closing the gap and merge in as little as 100 years.

That is an unusually short timescale by astronomical standards. It could give researchers a rare opportunity to investigate how supermassive black holes approach their final collision.

Gravitational waves could reveal an approaching merger

Mrk 501 is so distant that even the most powerful instruments available today cannot distinguish the two black holes as separate objects. The Event Horizon Telescope (EHT), which produced the first images of black holes in 2019 and 2022, also lacks the resolution needed to image this particular pair individually.

As a result, astronomers will not be able to watch the distance between the black holes decrease directly. Their approach could instead be detected through other signals.

As massive objects orbit one another, they generate gravitational waves—tiny distortions in spacetime that propagate outward through the universe. The proposed binary supermassive black hole system in Mrk 501 should produce very low-frequency gravitational waves that could be detected with pulsar timing arrays (PTAs).

PTAs use the highly regular radio pulses emitted by rapidly spinning neutron stars, known as pulsars, as precise cosmic clocks. By measuring subtle changes in the arrival times of these pulses on Earth, scientists can study the effects of passing gravitational waves.

Supermassive black hole binaries are already considered a leading explanation for the gravitational-wave background, a persistent signal believed to originate from many distant sources throughout the universe. In 2023, the European Pulsar Timing Array and other international collaborations reported evidence for this background.

The possible binary system in Mrk 501 makes the galaxy a particularly promising target for connecting gravitational-wave signals with an individual pair of supermassive black holes.

Detecting such a signal could do more than confirm the system’s existence. It could allow scientists to measure how the black holes’ orbits evolve and determine whether they are moving closer to an eventual merger.

“If gravitational waves are detected, we may be able to observe their frequency steadily increasing as the two giants spiral toward a collision, providing a unique opportunity to observe the unfolding of a supermassive black hole merger,” said co-author Hector Olivares.