Aligning Nuclear Cluster Orbits with an Active Galactic Nucleus Accretion Disk

TL;DR

This paper investigates how stars and remnants in nuclear star clusters can be captured by active galactic nucleus disks, potentially leading to observable gravitational wave events.

Contribution

It models the capture times of orbiting objects in AGN disks using drag forces, highlighting conditions for significant embedded populations.

Findings

Capture time depends on disk density, aspect ratio, and object velocity.

High-density disks with long lifetimes can embed many stellar objects.

Embedded objects may contribute to gravitational wave sources detectable by LIGO-Virgo and LISA.

Abstract

Active galactic nuclei (AGN) are powered by the accretion of disks of gas onto supermassive black holes (SMBHs). Stars and stellar remnants orbiting the SMBH in the nuclear star cluster (NSC) will interact with the AGN disk. Orbiters plunging through the disk experience a drag force and, through repeated passage, can have their orbits captured by the disk. A population of embedded objects in AGN disks may be a significant source of binary black hole mergers, supernovae, tidal disruption events and embedded gamma-ray bursts. For two representative AGN disk models we use geometric drag and Bondi-Hoyle-Littleton drag to determine the time to capture for stars and stellar remnants. We assume a range of initial inclination angles and semi-major axes for circular Keplerian prograde orbiters. Capture time strongly depends on the density and aspect ratio of the chosen disk model, the relative velocity of the stellar object with respect to the disk, and the AGN lifetime. We expect that for an AGN disk density $\rho \gtrsim 10^{-11}\rm g/cm^3$ and disk lifetime $\geq 1$Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.