Hubble spots a roaming black hole light-years from where it belongs

A black hole skulking in the shadows 600 million light-years away in space gave itself away with a dazzling flash, the light of a star it had just gnashed and eaten. 

Using NASA's Hubble Space Telescope and other observatories, astronomers found the cosmic object in an unexpected place. Rather than sitting dead center in its galaxy like most supermassive black holes, this one was thousands of light-years away from the core — 2,600, in fact. 

What's more, there is another enormous black hole that is the actual nucleus. While the catawampus black hole has the mass of 1 million suns, the one that defines the galactic center is 100 million times the mass of the sun. 

The burst of radiation detected, known as a tidal disruption event or TDE, began when a star wandered too close to the black hole. If not for that stellar snack, the black hole would have escaped astronomers' notice. 

"It opens up the entire possibility of uncovering this elusive population of wandering black holes with future sky surveys," said study author Yuhan Yao of UC Berkeley in a statement. "I think this discovery will motivate scientists to look for more examples of this type of event."

The Hubble Space Telescope, a partnership of NASA and the European Space Agency, confirms the presence of a wandering supermassive black hole, 600 million light-years from Earth. Credit: NASA

Out of about 100 TDE events discovered through surveys so far, this one, dubbed AT2024tvd, is the first scientists have seen emerging from a supermassive black hole that is not a galactic nucleus. The research team's findings, announced by NASA, will be published in an upcoming issue of The Astrophysical Journal Letters.

Black holes are some of the most inscrutable phenomena in outer space. About 50 years ago, they were little more than a theory — a kooky mathematical answer to a physics problem. Even astronomers at the top of their field weren't entirely convinced they existed. Today, not only are black holes accepted science, they're getting their pictures taken by a collection of enormous, synced-up radio dishes on Earth. 

Unlike a planet or star, black holes don't have surfaces. Instead, they have a boundary called an "event horizon," or a point of no return. If anything swoops too close, like the doomed aforementioned star, it will fall in, never to escape the hole's gravitational clutch.

The most common kind, called a stellar black hole, is thought to be the result of an enormous star dying in a supernova explosion. The star's material then collapses onto itself, condensing into a relatively tiny area. 

How supermassive black holes form is even more elusive. Astrophysicists believe these invisible giants lurk in the heart of virtually all galaxies. Recent Hubble observations have bolstered the theory that they begin in the dusty cores of starburst galaxies, where new stars are rapidly assembled, but scientists are still teasing that out. 

A supermassive black hole is off-center in a galaxy 600 million light-years from Earth. Credit: NASA / ESA / STScI / Ralf Crawford illustration

As the star was stretched and torn asunder in the TDE, some of its gas formed a glowing ring around the black hole. The resulting flare flashed brightly in ultraviolet and visible light.

Telescopes on the ground, such as the Zwicky Transient Facility in California, first detected it. But it was Hubble that confirmed the flare's off-center location. NASA’s Chandra X-ray Observatory in space and the Very Large Array in New Mexico provided supporting data.

The two supermassive black holes both reside in the same galaxy, yet they are not a binary pair, meaning they're not bound to each other through gravity. Scientists don't know how the wandering black hole got there. 

A star's remnants form a disk around a hidden supermassive black hole. Credit: NASA / ESA / STScI / Ralf Crawford illustration

One possibility is that the smaller black hole came from a smaller galaxy that at some point merged with the larger one, bringing its central black hole along for the ride. Eventually, the smaller black hole may spiral into the larger one. For now, it's doing its own thing. 

Another possibility is that it was ganged up on by a couple of bully black holes. In so-called three-body interactions, the lowest-mass object can be evicted from the center of a galaxy, with the two others remaining in the galaxy's core. 

"Theorists have predicted that a population of massive black holes located away from the centers of galaxies must exist," said Ryan Chornock, a member of the ZTF team, in a statement, "but now we can use TDEs to find them.”