For a long time, intermediate-mass black holes remained a mystery to astronomers. Now, however, they have evidence of one moving through the dwarf galaxy UGCA 320. What is more, they have obtained evidence that it is also actively consuming matter as it moves.
Wandering Black Hole in a Dwarf Galaxy
Astronomers have discovered the first direct evidence that a wandering black hole can feed by drawing gas into its wake as it moves through a galaxy. As reported by phys.org, this is the first direct evidence of an accretion channel that had long been predicted theoretically but had never previously been observed.
The black hole studied in this research, led by Xin Li of Westlake University in China, is located in UGCA 320, a dwarf irregular galaxy viewed edge-on from Earth and situated about 20 million light-years away. An image taken by the Hubble Space Telescope showed that the object lies outside the galaxy’s main star-forming disk.
Observations conducted with the MUSE instrument in 2021 revealed broad Balmer emission, a sign of a massive black hole accreting matter. The broad Balmer emission-line component made it possible to determine that the black hole has a mass of approximately 35,000 solar masses. Multiple independent observations support its identification as an accreting intermediate-mass black hole.
The characteristics of this “wandering” black hole match expectations. Intermediate-mass black holes have masses ranging from 100 to 100,000 times that of the Sun. They are believed to be the seeds of supermassive black holes found at the centers of galaxies. Some of them are expected eventually to drift far away from the gas-rich centers that normally feed them.
Unlike black holes at galactic centers, wandering black holes have limited access to mechanisms that can channel gas toward them, such as galaxy mergers, tidal interactions, cloud collisions, and gas cooling. Therefore, how these intermediate-mass black holes eventually grow into supermassive ones, reaching masses millions to billions of times greater than that of the Sun, remains a mystery.
The Gravitational “Wake” of an Intermediate-Mass Black Hole
There is a hypothetical mechanism that may explain how they can grow. “One plausible accretion channel for a wandering black hole is the gravitational wake it creates as it moves through the interstellar medium; this process is known as Bondi–Hoyle–Lyttleton (BHL) accretion,” the team writes in the paper.
This scenario proposes that when a wandering black hole moves through the gas filling its host galaxy, its gravity pulls nearby gas particles toward it. Gas gravitationally attracted to the black hole from different directions converges and accumulates into a denser stream trailing behind it — this denser region extending in its wake is the “wake.” Under this scenario, the black hole can feed on the captured gas.
This “gravitational focusing” also creates a bow shock in front of the black hole. The surrounding gas is expected to develop an asymmetric flow structure consisting of several gas components.
The team then decided to test whether this was actually the case for the wandering black hole in UGCA 320 using spectroscopic observations. Their analysis revealed all three components predicted by theory: low-density gas ahead of it, denser gas behind it, and dense clumps following the accretion flow.
Gas Obscures the Black Hole from Our View
The team found another clue in the way the black hole’s appearance changed over time. Spectroscopic observations showed that the broad hydrogen emission lines that were clearly visible in 2021 had almost disappeared by June 2025. In July 2025, they partially reappeared, only to disappear again by April 2026. This suggests that dense gas clumps embedded in the black hole’s accretion flow may have periodically crossed our line of sight, obscuring the region from which this emission originates.
The astronomers’ discovery provides observational evidence that wandering intermediate-mass black holes can actively accrete matter through gravitational wakes. This newly discovered “mobile” accretion pathway may offer an important clue as to how these black holes grow before eventually sinking toward the centers of galaxies and becoming supermassive black holes.
