Black hole binary systems embedded in AGN discs have been proposed as a source of the observed gravitational waves (GWs) from LIGO-Virgo-KAGRA. Studies have indicated binary-single encounters could be common place within this population, yet we lack a comprehensive understanding of how the ambient gas affects the dynamics of these three-body encounters. We present the first hydrodynamical simulations of black hole binary-single encounters in an AGN disc. We find gas is a non-negligible component of binary-single interactions, leading to unique dynamics, including the formation of quasi-stable hierarchical triples. The gas efficiently and reliably dissipates the energy of the three-body system, hardening the triple provided it remains bound after the initial encounter. The hardening timescale is shorter for higher ambient gas densities. Formed triple systems can be hardened reliably by $2-3$ orders of magnitude relative to the initial binary semi-major axis within less than a few AGN orbits, limited only by our resolution. We calculate that the gas hardening of the triple enhances the probability for a merger by a minimum factor of $3.5-8$ depending on our assumptions. In several cases, two of the black holes can execute periapses on the order of less than $10$ Schwarzschild radii, where the dynamics were fully resolved for previous close approaches. The likelihood of these prompt mergers increases when the gas density is larger. Our results suggest that current timescale estimates (without gas drag) for binary-single induced mergers are an upper bound. The shrinkage of the triple by gas has the prospect of increasing the chance for unique GW phenomena such as residual eccentricity, dephasing from a third object and double GW mergers.
Radiation hydrodynamic simulation of the Haro 11 galaxy: the escape of LyC and Ly$α$ in a dwarf galaxy merger
Oskar Klein Centre, Department of Astronomy, Stockholm University（斯德哥尔摩大学天文学系奥斯克·克莱因中心）；Centre de Recherche Astrophysique de Lyon UMR5574, Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS（里昂天体物理研究中心）；Lund Observatory, Division of Astrophysics, Department of Physics, Lund University（隆德大学物理系天体物理学部隆德天文台）
Set them free: extending RAMCOAL to model massive black hole triplets in hydrodynamical simulations of galaxies
Center for Computational Astrophysics, Flatiron Institute（平顿研究所计算天体物理学中心）；Institut d’astrophysique de Paris, UMR 7095, CNRS, Sorbonne Université（巴黎天体物理研究所，法国国家科学研究中心，索邦大学）；Institute for Astronomy, University of Edinburgh（爱丁堡大学天文研究所）
