Supermassive black holes sit at the centers of nearly all large galaxies, tipping the cosmic scales at millions or even billions of times the mass of the Sun. How they get that big remains a stubborn mystery: accreting gas from their surroundings is one route, but it's apparently too slow to account for the biggest examples. Merging with other massive black holes is the leading candidate for the missing explanation.
Galaxy collisions are common throughout the universe, so their central black holes ought to eventually come together too. As two galaxies merge, their black holes are expected to move closer, begin orbiting one another, and gradually spiral inward until they combine. Modeling those final stages, however, remains a headache, and astronomers have never reliably identified a pair of supermassive black holes orbiting extremely close together - despite abundant evidence of colliding galaxies.
Now an international research team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has uncovered direct observational evidence suggesting exactly such a pair at the center of the galaxy Mrk 501. The findings have been accepted for publication in Monthly Notices of the Royal Astronomical Society and are scheduled for an upcoming issue.
Astronomers have long known that Mrk 501's central black hole fires an intense jet of particles into space at nearly the speed of light, powered by the extreme conditions and powerful magnetic fields surrounding the black hole. To investigate the system, researchers examined detailed radio observations at multiple frequencies, collected on dozens of separate days over approximately 23 years. The extensive dataset revealed something remarkable: alongside the familiar jet was a second jet whose movement could also be tracked.
The observations provide what the researchers call the first direct imaging evidence of such a system at the center of a galaxy, strongly suggesting a second supermassive black hole is present. The two black holes themselves remain too close together in the sky to be individually resolved.
"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," Britzen reports.
Two Supermassive Black Holes in a Cosmic Dance
The previously known jet is aimed toward Earth, making it appear exceptionally bright and relatively easy to observe. The newly identified jet points in a different direction, which helps explain why it stayed hidden for so long.
Over just a few weeks of observations, astronomers noticed dramatic changes in the second jet's position. It appeared to emerge from behind the larger black hole and sweep counterclockwise around it. The movement repeated, suggesting the entire system was undergoing a regular orbital motion.
"Evaluating the data felt like being on a ship. The entire jet system is in motion. A system of two black holes can explain this: The orbital plane sways," Britzen explains.
The 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 in a way that creates a ringlike appearance. One possible explanation: the two black holes and Earth were briefly aligned in just the right way, with the gravity of the foreground black hole acting like a lens, bending light from the second jet behind it. This effect, called gravitational lensing, offers another potential clue to the system's nature.
Two Black Holes Could Merge in Just 100 Years
By examining changes in the jets and repeating patterns in their brightness, the team calculated that the two black holes appear to complete an orbit around each other approximately every 121 days. Their estimated separation is just 250 to 540 times the distance between Earth and the Sun - which sounds enormous, until you remember these objects carry masses ranging from 100 million to a billion times that of our Sun.
Such a tight orbit could have dramatic consequences. Depending on the black holes' actual masses, their separation may shrink rapidly enough for them to merge in as little as 100 years. That would be an exceptionally short countdown by astronomical standards, potentially giving researchers an extraordinary opportunity to investigate how supermassive black holes approach their final collision.
Gravitational Waves Could Reveal an Approaching Merger
Unfortunately, Mrk 501 is so distant that even today's most powerful instruments 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 capture this particular pair individually. So astronomers won't be able to directly watch the distance between the black holes shrink. Another method, however, could reveal their approach.
As massive objects orbit one another, they generate gravitational waves - tiny distortions in spacetime that travel outward through the universe. The proposed pair in Mrk 501 should produce extremely low-frequency gravitational waves detectable using pulsar timing arrays (PTAs). These arrays use the remarkably regular radio pulses emitted by rapidly rotating neutron stars, known as pulsars, as precise cosmic clocks. By measuring subtle changes in when those pulses arrive at Earth, scientists can search for the influence of passing gravitational waves.
Supermassive black hole binaries (SMBHBs) are already considered the leading explanation for the gravitational wave background, a persistent signal thought to arise from many distant sources across the universe. In 2023, the European Pulsar Timing Array and other research collaborations reported evidence of this background. The discovery in Mrk 501 now makes the galaxy a particularly promising target for connecting a gravitational wave signal to an individual pair of supermassive black holes.
Such a detection could do more than confirm the system's existence. It might allow scientists to measure how the pair's orbit evolves and determine whether the black holes are approaching their final merger.
"If gravitational waves are detected, 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," notes co-author Héctor Olivares.
Materials provided by Max Planck Institute for Radio Astronomy. Note: Content may be edited for style and length.