Black-hole mergers in disklike environments could explain the observed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>q</mml:mi><mml:mo>−</mml:mo><mml:msub><mml:mi>χ</mml:mi><mml:mi>eff</mml:mi></mml:msub></mml:math> correlation
Alessandro Santini, Davide Gerosa, R. Cotesta, Emanuele Berti
Abstract
Current gravitational-wave data from stellar-mass black-hole binary mergers suggest a correlation between the binary mass ratio $q$ and the effective spin ${\ensuremath{\chi}}_{\mathrm{eff}}$: 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, disklike 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.