2025ApJ...994L..47S

It remains an open question whether the binary black hole (BBH) mergers observed with gravitational-wave detectors originate from the evolution of isolated massive binary stars or were dynamically driven by perturbations from the environment. Recent evidence for nonzero orbital eccentricity in a handful of events is seen as support for a nonnegligible fraction of the population experiencing external driving of the merger. However, it is unclear from which formation channel eccentric BBH mergers would originate: dense star clusters, hierarchical field triples, active galactic nuclei, or wide binaries in the galaxy could all be culprits. Here, we investigate whether the spin properties of eccentric mergers could be used to break this degeneracy. Using the fact that different formation channels are predicted to either produce eccentric mergers with mutually aligned or randomly oriented black hole spins, we investigate how many confident detections would be needed in order for the two models to be statistically distinguishable. If a few percent of BBH mergers retain measurable eccentricity in the bandwidth of ground-based detectors, we report a ∼9% chance that we could confidently distinguish both models (Bayes factor ${\mathrm{ln}}\,{ \mathcal B }\gt 3$) after the fifth observing run of the LIGO-Virgo-KAGRA detector network, ∼63% for LIGO A#, and ∼98% for the Einstein Telescope and Cosmic Explorer.