Aligning Nuclear Cluster Orbits with an Active Galactic Nucleus Accretion Disk

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 ρ 10 −11 g/cm 3 and disk lifetime ≥ 1Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.

Stellar Evolution in AGN Disks

“…This interaction results in energy and momentum loss which, over many passages, can bring a star into a circular orbit corotating with the disk (Syer et al 1991;Artymowicz et al 1993). This trapping process relies on hydrodynamical drag as well as the excitation of resonant density waves and bending waves, and can be an efficient mechanism for r 10 pc (Artymowicz et al 1993;Just et al 2012;Kennedy et al 2016;MacLeod & Lin 2020;Fabj et al 2020). Since the stellar density is expected to be high in these regions (n * ∼ 10 6 pc −3 ) a large number of stars can potentially be trapped during periods of AGN activity.…”

Electromagnetic Signatures of Relativistic Explosions in AGN Disks

“…with the profiles ρ 0 (r) (the density in the disk mid plane) and H(r) (the disc scale-height) provided by the AGN models referenced above (see Fig. 1 in Fabj et al (2020) for a visual comparison between the main properties of these disks).…”

“…Paczynski 1978;Goodman 2003;Dittmann & Miller 2020), and capture from the nuclear star cluster surrounding the AGN (e.g. Artymowicz et al 1993;Fabj et al 2020), as a result of momentum and energy loss as the stars interact with the disk.…”