ABSTRACT Merging black holes (BHs) are expected to produce remnants with large dimensionless spin parameters (aspin ∼ 0.7). However, gravitational wave (GW) observations with LIGO–Virgo–Kagra (LVK) suggest that merging BHs are consistent with modestly positive but not high spin (aspin ∼ 0.2), causing tension with models suggesting that high-mass mergers are produced by hierarchical merger channels. Some BHs also show evidence for strong in-plane spin components. Here, we point out that spin-down of BHs due to eccentric prograde post-merger orbits within the gas of an active galactic nucleus (AGN) disc can yield BHs with masses in the upper mass gap, but only modestly positive aspin, and thus observations of BHs with low spin do not rule out hierarchical models. We also point out that the fraction of binary black hole (BBH) mergers with significant in-plane spin components is a strong test of interactions between disc BBHs and nuclear spheroid orbiters. Spin magnitude and spin tilt constraints from LVK observations of BBHs are an excellent test of dynamics of BHs in AGN discs, disc properties, and the nuclear clusters interacting with AGNs.

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