Model explains how black holes can be flung from galaxies
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- Pieter van Dokkum at Yale and colleagues identified in 2023 a thin, line-like shape in a Hubble Space Telescope image ~7.5 billion light years from Earth, concluding (with JWST follow-up) it was a supermassive black hole flying out of a galaxy at supersonic speeds, leaving a chain of young stars in its wake.
- Tousif Islam at UC Santa Barbara and colleagues built a mathematical model tracing a runaway black hole's origin, finding it would require two parent supermassive black holes — one spinning quickly and six times more massive than the other — that merged ~70 million years ago.
- The team estimated a large enough recoil to match the proposed runaway black hole would occur in fewer than 10% of supermassive black hole mergers, with most mergers producing much smaller kicks.
- Jorge Sánchez Almeida at the Institute of Astrophysics of the Canary Islands and colleagues have argued since 2023 that the line represents a galaxy with star-forming regions viewed edge-on, noting the mass-velocity relationship fits the Tully-Fisher relation.
- The original team stands by the runaway black hole interpretation, calling it the "least unfavourable" explanation among competing options, with Islam saying he became convinced of its plausibility after a black hole conference at his university.
- The debate is resolvable by JWST further imaging the region to test the edge-on galaxy hypothesis, or by detecting predicted gravitational waves from the parents' merger using the Laser Interferometer Space Antenna (LISA), according to the researchers.
Why it matters: If the runaway interpretation holds, it documents a rare mechanism — occurring in fewer than 10% of supermassive black hole mergers — for ejecting billion-solar-mass objects across intergalactic space. A competing edge-on galaxy reading remains unresolved, with JWST follow-up and future LISA gravitational wave detections positioned to distinguish between the two hypotheses.
