  Active galactic nuclei (AGN) are promising environments for the assembly of
merging binary black hole (BBH) systems. Interest in AGNs as nurseries for
merging BBH is rising following the detection of gravitational waves from a BBH
system from the purported pair-instability mass gap, most notably, GW190521.
Active galactic nuclei have also been invoked to explain the formation of the
high-mass-ratio system, GW190814. We draw on simulations of BBH systems in AGN
to propose a phenomenological model for the distribution of black hole spins of
merging binaries in AGN disks. The model incorporates distinct features that
make the AGN channel potentially distinguishable from other channels, such as
assembly in the field and in globular clusters. The model parameters can be
mapped heuristically to the age and density of AGN disks. We estimate the
extent to which different populations of mergers in AGNs can be distinguished.
If most merging black holes are assembled in AGNs, future gravitational-wave
observations may provide insights into the dynamics of AGN disks.
