Can stellar-mass black hole growth disrupt disks of active galactic nuclei? The role of mechanical feedback

TL;DR

This paper investigates how mechanical feedback from stellar-mass black holes in AGN disks can regulate their growth and influence disk dynamics, potentially resolving over-growth issues and explaining observed AGN properties.

Contribution

It introduces a feedback mechanism involving winds and jets that reduces black hole accretion rates and impacts AGN disk structure, addressing over-growth and observational discrepancies.

Findings

Feedback reduces sBH accretion rates by a factor of 10-100.

Cocoons of jets can unbind gas in less massive SMBH disks.

Mechanisms help explain the scarcity of high-Eddington ratio AGNs with low-mass SMBHs.

Abstract

Stellar-mass BHs (sBHs) are predicted to be embedded in active galactic nuclei (AGN) disks due to gravitational drag and in-situ star formation. However, we find that due to a high gas density in an AGN disk environment, compact objects may rapidly grow to intermediate-mass BHs and deplete matter from the AGN disk unless accretion is suppressed by some feedback process(es). These consequences are inconsistent with AGN observations and the dynamics of the Galactic center. Here we consider mechanical feedback mechanisms for the reduction of gas accretion. Rapidly accreting sBHs launch winds and/or jets via the Blandford-Znajek mechanism, which produce high-pressure shocks and cocoons. Such a shock and cocoon can spread laterally in the plane of the disk, eject the outer regions of a circum-sBH disk (CsBD) and puncture a hole in the AGN disk with horizontal size comparable to the disk scale-height. Since the depletion timescale of the bound CsBD is much shorter than the resupply timescale of gas to the sBH, the time-averaged accretion rate onto sBHs is reduced by this process by a factor of $\sim 10$--$100$. This feedback mechanism can therefore help alleviate the sBH over-growth and AGN-disk depletion problems. On the other hand, we find that cocoons of jets can unbind a large fraction of the gas accreting in the disks of less massive SMBHs, which may help explain the dearth of high-Eddington ratio AGNs with SMBH mass $\lesssim10^5{\rm M_\odot}$.