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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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LIGO–Virgo correlations between mass ratio and effective inspiral spin: testing the active galactic nuclei channel
ABSTRACT Observations by LIGO–Virgo of binary black hole mergers suggest a possible anticorrelation between black hole mass ratio (q = m2/m1) and the effective inspiral spin parameter χeff, the mass-weighted spin projection on to the binary orbital angular momentum. We show that such an anticorrelation can arise for binary black holes assembled in active galactic nuclei (AGNs) due to spherical and planar symmetry-breaking effects. We describe a phenomenological model in which (1) heavier black holes live in the AGN disc and tend to spin-up into alignment with the disc; (2) lighter black holes with random spin orientations live in the nuclear spheroid; (3) the AGN disc is dense enough to rapidly capture a fraction of the spheroid component, but small in radial extent to limit the number of bulk disc mergers; (4) migration within the disc is non-uniform, likely disrupted by feedback from migrators or disc turbulence; (5) dynamical encounters in the disc are common and preferentially disrupt binaries that are retrograde around their centre of mass, particularly at stalling orbits, or traps. Comparisons of predictions in (q, χeff) parameter space for the different channels may allow us to distinguish their fractional contributions to the observed merger rates.
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- Award ID(s):
- 2012057
- PAR ID:
- 10368329
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 514
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 3886-3893
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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