Rapid AGN accretion from counter-rotating discs

TL;DR

This paper investigates how counter-rotating or misaligned accretion discs around supermassive black holes can significantly enhance accretion rates, potentially explaining rapid black hole growth and SMBH binary coalescence.

Contribution

It provides an analytical estimate and SPH simulation results showing that tilted accretion discs can increase accretion rates by over two orders of magnitude.

Findings

Accretion rates can be enhanced by factors >100 in tilted discs.

Counter-rotation and misalignment facilitate rapid SMBH growth.

Results support chaotic accretion as a mechanism for SMBH evolution.

Abstract

Accretion in the nuclei of active galaxies may occur chaotically. This can produce accretion discs which are counter-rotating or strongly misaligned with respect to the spin of the central supermassive black hole (SMBH), or the axis of a close SMBH binary. Accordingly we consider the cancellation of angular momentum in accretion discs with a significant change of plane (tilt) between inner and outer parts. We estimate analytically the maximum accretion rate through such discs and compare this with the results of Smoothed Particle Hydrodynamics (SPH) simulations. These suggest that accretion rates on to supermassive black holes may be larger by factors $\gtrsim 100$ if the disc is internally tilted in this way rather than planar. This offers a natural way of driving the rapid growth of supermassive black holes, and the coalescence of SMBH binaries.