How to Escape from a Trap: Outcomes of Repeated Black Hole Mergers in Active Galactic Nuclei

Stellar-mass black holes (BHs) embedded in active galactic nuclei (AGN) may be major sources of astrophysical gravitational waves (GWs), contributing both to the observed LIGO-Virgo-KAGRA population of binary BH mergers and to future populations of LISA-band extreme mass ratio inspirals (EMRIs). The ability of these BHs to pair up into binaries, inspiral, and produce GWs will be shaped by the existence of migration traps, regions in the AGN where hydrodynamic torques vanish. Previous works have studied the existence and location of migration traps in AGN disks. Here, we investigate how individual BHs may escape such traps as an outcome of mergers, potentially suppressing hierarchical growth. We find that while GW recoil kicks are strong enough to kick merged BHs onto inclined orbits, gas drag quickly realigns them into the AGN disk. A more robust escape mechanism is gap opening: once a BH grows above a critical mass, its gravity disturbs the AGN gas sufficiently to eliminate the trap. In low-mass AGN relevant for LISA, gaps open easily, and the resulting “wet EMRI” masses are unlikely to reflect protracted hierarchical mergers. In combination with our previous work, we find that migration traps only exist in a relatively narrow range of AGN luminosities, L ∈ [10^43.5, 10^45.5] erg s⁻¹. We identify an even narrower AGN luminosity range for which stellar mass BHs can grow into the pair-instability mass gap and beyond. This characteristic luminosity scale may assist in indirect tests of the “AGN channel” for binary BH mergers.