Astronomers have identified the first direct evidence that a wandering black hole can sustain itself by drawing in gas through a gravitational wake as it travels within its galaxy. This discovery confirms a theoretical accretion process that had not been observed until now. The study, led by Xin Li of Westlake University in China, focuses on a black hole located in the dwarf irregular galaxy UGCA 320, approximately 20 million light-years from Earth. Unlike black holes anchored at galactic centers, this intermediate-mass black hole, estimated to be about 35,000 times the mass of the sun, resides outside the galaxy's main star-forming disk.
Intermediate-mass black holes, with masses between 100 and 100,000 solar masses, are believed to be precursors to supermassive black holes found at the centers of galaxies. However, how these wandering black holes grow without the typical gas-rich environments at galactic centers has remained unclear. The team investigated the possibility that the black hole accretes gas through a gravitational wake formed as it moves through the interstellar medium, a process known as Bondi–Hoyle–Lyttleton accretion.
In this scenario, the black hole's gravity pulls gas particles from its surroundings, creating a denser trailing stream or wake behind it. This wake serves as a reservoir of gas that the black hole can feed on. The gravitational focusing also generates a bow shock ahead of the black hole, resulting in a distinctive flow pattern of gas components around it.
Using spectroscopic data, the researchers confirmed the presence of three predicted gas components: low-density gas ahead of the black hole, denser gas trailing behind, and dense clumps tracing the accretion flow. Additionally, variations in the black hole's hydrogen emission lines over time suggest that dense gas clumps intermittently obscure the emission region, indicating dynamic accretion activity.
This observation provides crucial evidence that wandering intermediate-mass black holes can actively grow by accreting gas through gravitational wakes. Understanding this mobile accretion mechanism offers new insights into how such black holes may evolve into supermassive black holes as they eventually migrate toward galactic centers.