Gas Assisted Binary Black Hole Formation in AGN Discs

We investigate close encounters by stellar mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGN) as a potential formation channel of binary black holes (BBHs). We perform a series of 2D isothermal viscous hydrodynamical simulations within a shearing box prescription using the Eulerian grid code Athena++. We co-evolve the embedded BHs with the gas keeping track of the energetic dissipation and torquing of the BBH by gas gravitation and inertial forces. To probe the dependence of capture on the initial conditions, we discuss a suite of 345 simulations spanning local AGN disc density ($\rho_0$) and impact parameter ($b$) space. We identify a clear region in $b - \rho_0$ space where gas assisted BBH capture is efficient. We find that the presence of gas leads to strong energetic dissipation during close encounters between unbound BHs, forming stably bound eccentric BBHs. We find that the gas dissipation during close encounters increases for systems with increased disc density and deeper periapsis passages $r_p$, fitting a power law such that $\Delta E \propto \rho_0^{\alpha}r_p^{\beta}$ where $\{\alpha,\beta\} = \{1.01\pm0.04,-0.43\pm0.03\}$. Alternatively, the gas dissipation is approximately $\Delta E = 4.3 M_\text{d} v_\text{H} v_p$, where $M_\text{d}$ is the mass of a single BH minidisc just prior to the encounter when the binary separation is $2r_\text{H}$ (two binary Hill radii), $v_\text{H}$ and $v_p$ are the relative BH velocities at $2r_\text{H}$ and at the first closest approach, respectively. We derive a prescription for capture which can be used in semi-analytical models of AGN. We do not find the dissipative dynamics observed in these systems to be in agreement with the simple gas dynamical friction models often used in the literature.