Supermassive black holes are thought to exist at the centers of nearly all large galaxies. These enormous objects can weigh millions or even billions of times as much as the Sun, but scientists still do not fully understand how they grow to such extraordinary sizes. Pulling in (accreting) gas from their surroundings is one way they gain mass, although this process alone may be too slow to explain the largest examples.
Merging with other massive black holes could provide the missing explanation. Galaxy collisions are common throughout the universe, making it likely that their central black holes also eventually come together. As two galaxies merge, their black holes are expected to move closer, begin orbiting one another, and gradually spiral inward until they combine into a single object.
Despite this expectation, scientists still struggle to model the final stages of these mergers. Even more puzzling, astronomers have not reliably identified a pair of supermassive black holes orbiting extremely close together, despite abundant evidence of colliding galaxies. A Second Jet Reveals a Hidden Black Hole An international research team led by Silke Britzen of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has now uncovered direct observational evidence suggesting that such a pair exists 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 to appear in an upcoming issue. Astronomers have long known that the central black hole in Mrk 501 produces an intense jet of particles that shoots into space at nearly the speed of light. These jets arise from the extreme conditions surrounding black holes, where powerful magnetic fields help accelerate material outward.
To investigate the system, researchers examined detailed radio observations collected at multiple frequencies 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 describe as the first direct imaging evidence of such a system at the center of a galaxy, strongly suggesting that a second supermassive black hole is present.
The black holes themselves, however, 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," reports Silke Britzen. 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.
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