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Schematic illustration of Bondi–Hoyle–Lyttleton accretion onto a wandering intermediate-mass black hole. In the rest frame of the intermediate-mass black hole (IMBH), the ambient gas flows from the upstream region (blue, left) towards the downstream region (orange, right), as indicated by the solid streamlines. The captured gas circulates around the black hole before eventually being accreted (white dashed curve), and partially covers the central emitting region along the observer's line of sight (gray dashed line). Credit: arXiv (2026). DOI: 10.48550/arxiv.2608.10719
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.
Pan-STARRS false-color image of UGCA 320, with the MUSE field of view (FOV) outlined in red. The white arrow indicates the direction of the paired galaxy UGCA 319, while the yellow arrow marks the position of the discovered wandering intermediate-mass black hole (IMBH), UGCA320-IMBH. b, HST false-color image of UGCA 320, centered on the MUSE FOV and overlaid with the WiFeS (tomato) and X-Shooter (magenta) FOVs. Credit: arXiv (2026). DOI: 10.48550/arxiv.2608.10719
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.
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