Binary Black Holes Embedded in AGN Disks
Stellar-mass binary black holes (BBHs) embedded in AGN disks are a promising channel for the mergers detected in gravitational waves by LIGO/Virgo/KAGRA, as these disks host abundant compact objects, favor hierarchical mergers, and may produce electromagnetic counterparts. I carried out some of the first high-resolution local (shearing-box) simulations of how embedded BBHs evolve, surveyed a wide parameter space, and identified the conditions under which their orbits harden.
A series of 2D high-resolution shearing-box simulations follows how embedded binaries evolve through their interactions with disk gas. Prograde circular comparable-mass binaries contract, and eccentric binaries undergo eccentricity damping (Paper I). The outcome depends on the equation of state and on the binary's mass and separation relative to the SMBH mass and Hill radius: non-isothermal, turbulent gas slows accretion, and less massive, wider binaries harden most efficiently (Paper II). Viscosity greatly raises accretion rates, so the positive accretion torque can dominate the gravitational torque, though the accreted gas carries less angular momentum per unit mass, which eases orbital contraction (Paper III).
Most embedded-binary studies use local shearing-box domains detached from the global disk. This paper provides a framework for assessing when that approximation holds in viscous hydrodynamic disks — identifying, for black hole masses of interest, the AGN disk radii beyond which global influences can be neglected across disk models — and, more generally, a systematic way to link local simulations to the global problem they aim to approximate and to gauge their accuracy.
Three-dimensional hydrodynamic simulations of circular binaries in a uniform gas flow show that the gas-driven torque arises from the gravitational coupling between each black hole and its companion's wake, controlled by a single parameter η — the ratio of gas flow speed to binary orbital speed. At small η (≲4) the wakes merge and the torque drives inspiral; at larger η they separate and the torque turns positive, driving expansion. In a standard AGN disk model, capture-channel binaries fall in the positive-torque regime, with expansion timescales shorter than the AGN disk lifetime, so gas-driven expansion can compete with gravitational-wave inspiral and suppress the capture-channel merger rate.