Migration Traps in Disks Around Supermassive Black Holes

TL;DR

This paper investigates how embedded objects in accretion disks around supermassive black holes can become trapped at certain radii due to changes in gas torque, impacting black hole growth and gravitational wave sources.

Contribution

It introduces an empirical analytic model for gas torques in SMBH disks, identifying specific radii where migration traps occur, which was not previously characterized.

Findings

Migration traps occur where the disk surface density gradient changes sign.

Traps are located around 20-300 Schwarzschild radii from the SMBH.

These traps facilitate black hole accumulation and potential intermediate mass black hole formation.

Abstract

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.