Astronomers have identified the first direct observational evidence that a wandering intermediate-mass black hole can accrete gas by dragging it along in a gravitational wake as it moves through its galaxy. This discovery confirms a theoretical accretion mechanism that had not been previously observed.
The black hole, with a mass approximately 35,000 times that of the sun, resides in UGCA 320, a dwarf irregular galaxy about 20 million light-years away. Unlike black holes located at galactic centers, this one is positioned outside the galaxy's main star-forming disk, limiting its access to typical gas-feeding processes such as galaxy mergers or tidal interactions.
Intermediate-mass black holes, ranging from 100 to 100,000 solar masses, are thought to be the precursors to supermassive black holes found at galaxy centers. However, how these wandering black holes grow without the usual gas supply channels has remained unclear.
The team, led by Xin Li of Westlake University, investigated whether the black hole could feed through a gravitational wake formed as it moves through the interstellar medium. This process, known as Bondi–Hoyle–Lyttleton accretion, predicts that the black hole's gravity pulls gas toward it, creating a denser trailing stream of gas behind it, along with a bow shock ahead.
Spectroscopic observations confirmed the presence of three gas components predicted by the theory: low-density gas in front, denser gas trailing behind, and dense clumps tracing the accretion flow. Additionally, variations in hydrogen emission lines over time suggest that dense gas clumps periodically obscure the emission region.
This finding provides important insight into how wandering intermediate-mass black holes can actively accrete material and potentially grow into supermassive black holes before settling into galactic centers.