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 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 ∼ 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 10 5 M ⊙ .

Top-heavy stellar mass distribution in galactic nuclei inferred from the universally high abundance ratio of [Fe/Mg]

“…These values are compatible with the low-mass peak in the mass spectra of merging BHs inferred from LIGO-Virgo observations, but do not explain the rate for heavier BBHs at 𝑀 • ∼ 30 M (Abbott et al 2021). Therefore, further mass growth processes of remnant BHs, such as continuous gas accretion and repeated mergers in dense AGN disks, are essential to simultaneously reproduce the chemical abundance ratio in BLRs and the mass distribution of merging BHs Safarzadeh & Haiman 2020;Tagawa et al 2020Tagawa et al , 2021Wang et al 2021). Finally, combining Eqs.…”

Spin Evolution of Stellar-mass Black Holes Embedded in AGN disks: Orbital Eccentricity Produces Retrograde Circumstellar Flows

“…where τ Sal = M • c 2 /L E = 5 × 10 8 yr is the Salpeter (1964) timescale, L E = 1.25 × 10 38 M • /M erg s −1 is the Eddington luminosity, and M • is specifically used to denote the companion sBH's mass. The accretion efficiency η, which also depends on black hole spin, is usually on the order of a few percent for isolated black holes (Jiang et al 2014(Jiang et al , 2019 or even much lower due to the strong outflow/jet for the embedded sBHs in AGN disks (Pan & Yang 2021;Tagawa et al 2021b).…”