UMD Astronomers Spot First ‘Wandering’ Black Hole

In a new study, University of Maryland astronomers spotted a supermassive black hole lurking at the edge of a galaxy—the first time scientists have detected a dormant black hole so far from a galactic center. Normally invisible, the celestial abyss revealed itself with a flash of light as it swallowed a star. The finding was published Monday in The Astrophysical Journal Letters.

Until now, astronomers have assumed that supermassive black holes reside in the centers of massive galaxies, said study co-author Suvi Gezari, an associate professor of astronomy at UMD. 

“This discovery will have a huge impact,” she said. “It means that we're going to find many more examples of wandering black holes, and we can understand how galaxies and their black holes merge and build up over time.”

Theorists have predicted that “wandering” black holes would roam the outskirts of colliding and merging galaxies. But because many black holes are quiescent—meaning they aren’t swallowing material or emitting light—they tiptoe around undetected by earthly sensors. 

The normally quiescent black hole described in this study was first detected through the Zwicky Transient Facility (ZTF), which monitors the sky for sudden changes in the cosmos, said study lead author Robert David Stein, a Neil Gehrels Prize Postdoctoral Fellow at the Joint Space-Science Institute, a research partnership between UMD’s Departments of Astronomy and Physics and NASA’s Goddard Space Flight Center (GSFC). 

ZTF scans the entire northern sky every two days using two telescopes based at the Palomar Observatory near San Diego. It’s impossible to check for black holes in each of the hundreds of thousands of celestial events that ZTF documents each night, so the researchers developed an artificial intelligence program that identifies a characteristic light pattern that occurs when a black hole shreds a star, known as a tidal disruption event. Those events are well-documented at galaxy centers, but the AI looks for similar flares anywhere in the sky. 

The UMD team launched its AI program in August 2025 and made its black hole discovery just three months later. “We weren't really sure we would be successful so quickly,” Stein said.

The black hole described in this study is located 9.3 kiloparsecs (about 30,000 light-years) away from the center of its galaxy. It’s about the same mass as the black hole at the center of our galaxy, the Milky Way. 

“To have such a big black hole outside of a galaxy is surprising to me,” said study co-author and UMD astronomy Professor Sylvain Veilleux. “There should be a Milky Way-like object around it, and that's definitely not the case.” 

This black hole likely originated from two colliding galaxies. One possibility is that a larger galaxy cannibalized a smaller one and then slowly stripped away its stars until only the core remained. A second possibility is that one galaxy already had two black holes orbiting closely at its center, called a binary black hole. If a third black hole from another merger enters the mix, complex three-body interactions would kick the smallest one out to wander at the galaxy edge. Continued observations of this tidal disruption event could favor one of these two theories over the other, the authors said. 

Characterizing how quiescent black holes behave is important because most black holes—including the one at the center of the Milky Way—are dormant. It’s not known if our galaxy hosts any wandering black holes on its outskirts, but Stein said we shouldn’t fear getting sucked into one: “We're very unlikely to meet one, at least in our lifetime.”

The team is now scanning the skies for more wandering black holes so they can “understand how galaxies form and how many black holes are whizzing around,” Stein said. This latest discovery shows that it’s possible to detect them without expensive techniques beyond standard sky surveys and machine learning.

“It is super exciting,” Gezari said. “It’s an example of machine learning opening up a whole new area of research.” 

Stein expects they’ll detect dozens or even hundreds of wandering black holes each year using the NSF-DOE Vera C. Rubin Observatory, which officially opened in June in Chile. The observatory provides extraordinarily detailed images using the world’s largest digital camera. 

Vellieux added that the Lowell Discovery Telescope and its Rapid infrared IMAger-Spectrometer—which debuted in June 2025 as a collaboration between GSFC, the UMD Department of Astronomy and Lowell Observatory—will allow astronomers to detect tidal disruption events from even greater distances from Earth than previously imagined. 

“This is the strongest case of a wandering black hole that we know,” Veilleux said. This is going to set the standard.”