Accretion and dynamical evolutions of stellar mass black holes in active galactic nucleus disks

<p indent="0mm">The merging of stellar-mass binary black holes within active galactic nucleus (AGN) accretion disks, referred to as the “AGN channel”, offers a distinctive dynamical pathway for gravitational wave sources detected by LIGO/Virgo/KAGRA. Stellar-mass black holes within AGN disks can form either through gravitational instability in the outer regions of the disks or via dynamical capture from the nuclear star cluster. These black holes, when they experience close encounters facilitated by the surrounding gas, can form binary black holes embedded in the AGN disks. The dense gaseous environment surrounding the embedded binary black hole in this channel can lead to significantly more massive merger events with detectable electromagnetic signatures. The discovery of the heaviest binary black hole merger event, GW 190521, within the mass gap regime and its potential electromagnetic counterparts, exemplifies a candidate for the AGN channel. However, the dynamical evolution and accretion processes of binary black holes in AGN disks remain largely unresolved due to the complex multi-scale physical processes involved. To understand this dynamical channel and the accretion processes of the binary black hole, multi-dimensional numerical simulations have been conducted. Drawing from the experience of numerical simulations of planet-disk interactions in protoplanetary disks, recent significant progress has been made in understanding the accretion, formation, and evolution of binary black holes in AGN disks. The accretion of embedded black holes is influenced by both the angular momentum supply from the background shear flow of the AGN disk and the tidal torque exerted by the embedded black hole on the global disk, which can lead to the formation of mini-disks around the embedded black holes. Due to the differential rotation of the background flow and gap opening by the binary, accretion onto binary black holes differs significantly from classical Bondi accretion. Although the accretion rate is considerably suppressed compared to Bondi accretion, it still greatly exceeds the Eddington accretion rate for stellar-mass black holes. Feedback from the embedded black holes can further suppress the accretion rates onto these stellar-mass black holes. Accurately accounting for the dynamical effects of the gaseous disk on binary evolution requires resolving the disk structure around the binary black holes, including both circum-single and circum-binary disk structures. The mini-disk structures around the binary black hole can significantly alter the dynamical evolution of the embedded binary, potentially causing the binary orbit to expand over time in certain conditions. Nevertheless, a broad parameter space exists where binary black holes can efficiently contract their orbits over time. Such orbital contraction scenarios include retrograde binary black holes, non-isothermal equations of state, equal mass binaries, smaller total mass of the binary black holes, and larger orbital separations of the binary black holes. There remain many open questions regarding the dynamical channel of embedded binary black holes in AGN disks. The effects of feedback from stellar-mass black holes and AGN disk turbulence have been largely neglected thus far, which could potentially modify the disk structures around the binary black holes and, consequently, the evolution of the embedded binary. Additionally, these factors could alter the accretion history of the stellar-mass binary black holes, which are crucial for predicting the electromagnetic counterparts of merging binary black holes in AGN disks.