Migration Traps in Disks Around Supermassive Black Holes
  Accretion disks around supermassive black holes (SMBHs) in active galactic
nuclei contain stars, stellar mass black holes, and other stellar remnants,
which perturb the disk gas gravitationally. The resulting density perturbations
in turn exert torques on the embedded masses causing them to migrate through
the disk in a manner analogous to the behavior of planets in protoplanetary
disks. We determine the strength and direction of these torques using an
empirical analytic description dependent on local disk gradients, applied to
two different analytic, steady-state disk models of SMBH accretion disks. We
find that there are radii in such disks where the gas torque changes sign,
trapping migrating objects. Our analysis shows that major migration traps
generally occur where the disk surface density gradient changes sign from
positive to negative, around 20--300$R_{\rm g}$, where $R_{\rm g}=2GM/c^{2}$ is
the Schwarzschild radius. At these traps, massive objects in the AGN disk can
accumulate, collide, scatter, and accrete. Intermediate mass black hole
formation is likely in these disk locations, which may lead to preferential gap
and cavity creation at these radii. Our model thus has significant implications
for SMBH growth as well as gravitational wave source populations.
