Migration traps in AGN disks and hierarchical mergers as promising origin of the observed q-χ_eff correlation

The most recent analyses of binary black hole detections by LIGO and Virgo have highlighted the presence of a correlation between the mass ratio q and the effective spin χ_eff. In particular, data suggest that unequal-mass binaries show systematically larger values of the effective spin. We propose that this observed feature could be interpreted using hierarchical black hole mergers. We explore this in the context of AGN disks where a single black hole accretes a series of smaller black holes captured in a migration trap. We develop a toy model where the key free parameter is the average black-hole generation. Remarkably, such a simple parametric model appears to reproduce, at least qualitatively, the observed correlation. Our results indicate that migration traps in AGN disks could represent a promising formation channel for (some of) the LIGO/Virgo events.