Astronomers have found the first direct evidence that a wandering black hole can feed itself by dragging gas along in its wake as it moves through its galaxy. It’s the first direct evidence of an accretion channel long predicted in theory but never before observed. The paper describing this discovery was posted to the arXiv preprint server on Aug. 11.
Mid-sized black holes
The black hole investigated in this study, led by Xin Li of Westlake University in China, is located in UGCA 320—an edge-on dwarf irregular galaxy about 20 million light-years away. A Hubble Space Telescope image showed that this object sat outside the galaxy’s main star-forming disk. MUSE observations from 2021 revealed broad Balmer emission, a signature of an accreting massive black hole. The broad Balmer emission-line component revealed the black hole’s mass to be around 35,000 times the sun’s mass. Multiple independent observations support its identification as an accreting intermediate-mass black hole.
The traits of this “wandering” black hole checked out. Intermediate-mass black holes have masses ranging from 100 to 100,000 times the sun’s mass. They are thought to be the seeds of the supermassive black holes found at galaxy centers. Some are expected to end up drifting far 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 funnel gas toward them, such as galaxy mergers, tidal interactions, cloud collisions and gas cooling. Therefore, how these intermediate-mass black holes end up growing into supermassive ones, reaching masses ranging from millions to billions of times the sun’s mass, remains a mystery.
A trailing ‘wake’
There is a proposed mechanism that predicts how they may grow. “One plausible accretion channel for a wandering black hole is through the gravitational wake it generates while moving through the interstellar medium, known as Bondi–Hoyle–Lyttleton accretion (BHL),” the team writes in the paper.
This scenario suggests that as the wandering black hole plows through the gas that fills its host galaxy, its gravity pulls nearby gas particles toward it. Gas gravitationally pulled toward the black hole from multiple directions converges and piles up into a denser trailing stream—this trailing, denser region is the “wake.” In this scenario, the black hole can feed from the captured gas.
This “gravitational focusing” also creates a bow shock in front of the black hole. The surrounding gas is expected to develop a lopsided flow structure consisting of multiple gas components.
The team went on to investigate whether this is the case for UGCA 320’s wandering black hole using spectroscopic observations. Their analysis revealed all three components predicted by the theory: low-density gas ahead of it, denser gas trailing behind and dense clumps tracing the accretion flow.
The team found another clue in the black hole’s changing appearance over time. Spectroscopic observations showed that the broad hydrogen emission lines, which were prominent in 2021, had almost disappeared by June 2025. They partially reappeared in July 2025 and faded again by April 2026. They suggest that dense clumps of gas embedded within the black hole’s accretion flow may have periodically moved into our line of sight, obscuring the region where these emissions originate.
“Our discovery provides the observational evidence that wandering intermediate-mass black holes can actively accrete through gravitational wakes,” the team concludes. They say that this newfound “mobile” accretion pathway may be an important clue to how they grow before they eventually sink into the centers of galaxies, transforming into supermassive black holes.
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Publication details
Xin Li et al, A Wandering 35,000-Solar-Mass Black Hole Fed by a Gravitational Wake, arXiv (2026). DOI: 10.48550/arxiv.2608.10719
Journal information:
arXiv
Key concepts
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A wandering black hole caught feeding on the run (2026, September 9)
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