Can Stellar-mass Black Hole Growth Disrupt Disks of Active Galactic Nuclei? The Role of Mechanical Feedback

Abstract

Stellar-mass BHs (sBHs) are predicted to be embedded in active galactic nucleus (AGN) disks owing 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 ∼10–100. This feedback mechanism can therefore help alleviate the sBH overgrowth 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 supermassive BHs (SMBHs), which may help explain the dearth of high-Eddington-ratio AGNs with SMBH mass ≲ 10 5 M ⊙ .