Scientists have begun to unravel the origin story of a cataclysmic collision between two black holes, which seem to have met their fate on a rarely observed 'squashed' orbital path.
The binary black hole merger event is one of only a few spotted by the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the US, and their Italian counterpart Virgo, with evidence for detectable eccentricity – an orbital shape more oval than circular close to the time of its merger.
The international team used simulations of three possible formation scenarios for the event, known as GW200208_222617, concluding it likely formed in a three-body star system or another densely populated system like a star cluster.
Their findings, presented in Physical Review D, could help scientists refine their techniques for pinpointing the evolutionary origins of binary black holes in the observable universe, revealing more about the environments in which these binaries meet and merge.
Lead author Dr Isobel Romero-Shaw, who recently joined Cardiff University's Gravity Exploration Institute after carrying out the research at the University of Bristol and the University of Cambridge, said: "Confidently detecting eccentricity is crucial for identifying how the binary black holes that we see with gravitational waves are forming."
Most binaries that have lived together for a long time will have circular orbits. If the orbit is squashed, or eccentric, it means the binary either became bound relatively recently ̶ perhaps the components only just met - or the orbit has been driven to higher eccentricity through interactions with external matter, like gas or a third object.
"By comparing the properties of GW200208_222617 to predictions from simulations of binary black holes evolving in field triples, dense star clusters, and active galactic nuclei, we find that it is much more likely to have formed in a field triple or dense cluster than an active galactic nucleus."
The event, detected as part of the LIGO-Virgo-KAGRA Collaboration's third observational run, was identified by multiple different groups using different analysis techniques as containing evidence of detectable orbital eccentricity.
By looking at its other properties and comparing them to predictions for various formation channels, the team started to narrow down how this binary – and therefore many others in the detected population – most likely formed.
"Eccentricity is 'smoking gun' evidence that a binary did not form in full isolation ̶ if it evolved on its own, it doesn't retain detectable eccentricity when it is observed with our current instruments," explains Dr Romero-Shaw, who is part of the 2025 cohort selected for the Science and Technology Facilities Council's Ernest Rutherford Fellowships.
"So, while we can't identify exactly which channel produced this merger yet, we're confident that if it is indeed eccentric, it wasn't formed in isolation. And so this begins to narrow down our options. This has implications for the rest of the population, because only a small fraction of binary black holes from each of these channels will be detectably eccentric.
"This means, if a single eccentric event is confidently detected, many more of the binary black holes that we've seen also likely evolved through the same formation channel. For example, if GW200208_222617 did indeed form in a dense cluster, then anywhere between about 7% and 100% of the rest of the population formed in a dense cluster environment."
The team hopes their study motivates gravitational waveform modellers to develop more advanced models, so that more confident detections of eccentricity can be achieved and the evolutionary origins of these kinds of binary black holes can be confirmed.
Their study, ' GW200208_222617 as an eccentric black-hole binary merger: properties and astrophysical implications ' is published in Physical Review D.
