Gas assisted binary black hole formation in AGN discs

Abstract

We investigate close encounters by stellar mass black holes (BHs) in the gaseous discs of active galactic nuclei (AGNs) 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 BBH impact parameter (b) and local AGN disc density (ρ0). We identify a clear region in b − ρ0 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 rp, fitting a power law such that $\Delta E \propto \rho _0^{\alpha }r_{\mathrm{p}}^{\beta }$, where {α, β} = {1.01 ± 0.04, −0.43 ± 0.03}. Alternatively, the gas dissipation is approximately ΔE = 4.3MdvHvp, where Md is the mass of a single BH minidisc just prior to the encounter when the binary separation is 2rH (two binary Hill radii), vH and vp are the relative BH velocities at 2rH 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.

Runaway Eccentricity Growth: A Pathway for Binary Black Hole Mergers in AGN Disks

“…The presence of gas around the merger may facilitate electromagnetic counterparts to these events (e.g., McKernan et al 2019;Tagawa et al 2023). The BBHs may be captured in the AGN disks from the nuclear star cluster (Bartos et al 2017), or form in situ via migration (e.g., Bellovary et al 2016;Secunda et al 2019;McKernan et al 2020b) or gas-assisted kinetic energy dissipation (Tagawa et al 2020;Rowan et al 2023Rowan et al , 2024Whitehead et al 2024). Regarding the latter, recently found that BBHs commonly form with large initial eccentricities and retrograde orbits, which was largely confirmed by Whitehead et al (2024).…”

“…We used the publicly available code Athena++ (Stone et al 2020) with the shearing box approximation (Hawley et al 1995;Stone et al 1996) and coupled it with the N-body code REBOUND (Rein & Liu 2012). With this, we accurately evolve arbitrary configurations of the three-body system without relying on ad hoc precession terms (e.g., or simplifying assumptions (e.g., Paper I; Whitehead et al 2024).…”

“…Similar to previous results (e.g., Paper I; Li & Lai 2022; Paper II), we have also found that the orbital eccentricity of prograde binaries will decay. Since BBHs formed in the dynamical environment of the AGN disk are expected to form with high orbital eccentricities (e.g., Rowan et al 2023;Whitehead et al 2024), understanding the rate of orbital decay versus eccentricity damping or pumping is important to make inferences about the binary when it enters GW detector bands. Prograde binaries have their eccentricities damped very slowly compared with their contraction rate when e b  0.…”

“…The BBHs may be captured in the AGN disks from the nuclear star cluster (Bartos et al 2017), or form in situ via migration (e.g., Bellovary et al 2016;Secunda et al 2019;McKernan et al 2020b) or gas-assisted kinetic energy dissipation (Tagawa et al 2020;Rowan et al 2023Rowan et al , 2024Whitehead et al 2024). Regarding the latter, recently found that BBHs commonly form with large initial eccentricities and retrograde orbits, which was largely confirmed by Whitehead et al (2024). Once formed, BBHs can be driven to coalescence by, for example, binarysingle interactions (Leigh et al 2018;Samsing et al 2022) or binary-disk interactions (Baruteau et al 2011).…”