From Inspiral to Expansion: The Wake-Driven Torque on Binary Black Holes in Gaseous Medium
Binary black holes (BBHs) in gaseous medium, such as active galactic nucleus (AGN) disks, are important gravitational-wave sources, yet the gas-driven torque that governs their orbital evolution remains to be fully understood. Most existing studies of BBHs in gas often approximate the net torque as the sum of independent dynamical friction (DF) forces exerted on each black hole by its own wake, neglecting the mutual gravitational coupling between the two wakes. We perform three-dimensional hydrodynamic simulations of circular BBHs in a uniform flow and find the torque is determined by the wake-wake interactions. The net torque is controlled by a single parameter $\eta \equiv v_g/v_o$, the ratio of the gas flow velocity to the binary orbital velocity. At small $\eta$, the wakes merge into a single overdense envelope and the time-averaged torque is negative; as $\eta$ increases, the wakes separate and the torque becomes positive. In AGN disks, capture-channel binaries in a disk model naturally produce $\eta$ in the positive-torque regime; the expansion timescale is comparable to or shorter than the disk lifetime, suggesting that gas-driven expansion can compete with gravitational-wave inspiral and suppress the capture-channel merger rate.