Supermassive black holes sit at the centers of most large galaxies, and they are some of the most powerful objects in the universe.
Scientists believe these giants grow over time by merging with other black holes, but catching them in the act has been extremely difficult.
Now, a team of astronomers has found strong evidence of two supermassive black holes orbiting each other very closely in a distant galaxy called Markarian 501.
This may be the first time such a tight pair has been observed just before merging.
The discovery was led by Silke Britzen from the Max Planck Institute for Radio Astronomy and published in Monthly Notices of the Royal Astronomical Society.
It offers a rare chance to study a key process in how galaxies evolve.
Black holes can grow by pulling in gas and dust, but that alone is not enough to explain how they become so massive—sometimes billions of times heavier than the sun.
Scientists think that when galaxies collide, their central black holes eventually meet, orbit each other, and finally merge into a single, even larger black hole.
While galaxy collisions are common over cosmic time, this final stage of black hole merging has been hard to observe directly.
The breakthrough came from studying Markarian 501, a galaxy in the constellation Hercules. At its center, a supermassive black hole shoots out a powerful jet of particles moving close to the speed of light.
By analyzing detailed radio observations collected over 23 years, the researchers noticed something unusual. Instead of just one jet, they found evidence of a second jet coming from a different direction.
This second jet was faint and harder to detect, but careful analysis revealed that it moves in a repeating pattern.
It appears to originate behind the first black hole and then swings around it in a circular motion. This strange behavior strongly suggests that there are actually two black holes orbiting each other, each contributing to the overall motion of the jets.
At one point in June 2022, the system produced an especially striking effect. The light from the jets appeared to form a ring shape, known as an Einstein ring.
This happens when gravity bends light in a very precise alignment. In this case, the front black hole likely acted as a lens, bending the light from the second jet behind it.
By tracking changes in the jets over time, the team estimated that the two black holes orbit each other every 121 days.
They are separated by a distance only a few hundred times the distance between Earth and the sun, which is extremely close for objects of such enormous mass. Depending on their exact sizes, the two could merge in as little as 100 years.
Even with the most advanced telescopes, scientists cannot directly see the two black holes as separate objects because they are so far away. However, their motion may soon be detected in another way. As they spiral closer together, they should produce gravitational waves—ripples in space-time—that could be picked up by special instruments on Earth.
If confirmed, this system could become one of the best examples of a supermassive black hole pair on the verge of merging, giving scientists a unique opportunity to watch one of the universe’s most dramatic events unfold.
