Evolution of Warped Accretion Disks in Active Galactic Nuclei. I. Roles of Feeding at the Outer Boundaries

TL;DR

This study explores how different feeding mechanisms at the outer boundaries of warped accretion disks influence the alignment and spin evolution of black holes in active galactic nuclei over million-year timescales.

Contribution

It introduces a detailed analysis of fixed versus free outer boundary feeding in warped disks, highlighting their distinct effects on black hole spin alignment and evolution.

Findings

Fixed feeding leads to consistent alignment within 1 million years.

Initial inclination angles greater than pi/2 cause a transition from retrograde to prograde accretion.

Free feeding allows for both alignment and anti-alignment, depending on initial conditions.

Abstract

We investigate the alignment processes of spinning black holes and their surrounding warped accretion disks in a frame of two different types of feeding at the outer boundaries. We consider (1) fixed flows in which gas is continually fed with a preferred angular momentum, and (2) free flows in which there is no gas supply and the disks diffuse freely at their outer edges. As expected, we find that for the cases of fixed flows the black hole disk systems always end up aligning on timescales of several 1e6 yr, irrespective of the initial inclinations. If the initial inclination angles are larger than pi/2, the black hole accretion transits from retrograde to prograde fashion, and the accreted mass onto the black holes during these two phases is comparable. On the other hand, for the cases of free flows, both alignments and anti-alignments can occur, depending on the initial inclinations and the ratios of the angular momentum of the disks to that of the black holes. In such cases, the disks will be consumed within timescales of 1e6 yr by black holes accreting at the Eddington limit. We propose that there is a close connection between the black hole spin and the lifetime for which the feeding persists, which determines the observable episodic lifetimes of active galactic nuclei. We conclude that careful inclusion of the disk feeding at the outer boundaries is crucial for modeling the evolution of the black hole spin.