Current gravitational-wave data from stellar-mass black-hole binary mergers suggest a correlation between the binary mass ratio q and the effective spin χeff: more unequal-mass binaries consistently show larger and positive values of the effective spin. Multiple generations of black-hole mergers in dense astrophysical environments may provide a way to form unequal-mass systems, but they cannot explain the observed correlation on their own. We show that the symmetry of the astrophysical environment is a crucial feature to shed light on this otherwise puzzling piece of observational evidence. We present a toy model that reproduces, at least qualitatively, the observed correlation. The model relies on axisymmetric, disk-like environments where binaries participating in hierarchical mergers share a preferential direction. Migration traps in AGN disks are a prime candidate for this setup, hinting at the exciting possibility of constraining their occurrence with gravitational-wave data.
@article{arxiv.2308.12998,
title = {Black-hole mergers in disklike environments could explain the observed $q$-$\chi_\mathrm{eff}$ correlation},
author = {Alessandro Santini and Davide Gerosa and Roberto Cotesta and Emanuele Berti},
journal= {arXiv preprint arXiv:2308.12998},
year = {2023}
}