Symmetry Breaking in Dynamical Encounters in the Disks of Active Galactic Nuclei

Active galactic nucleus (AGN) disks may be important sites for stellar mass binary black hole (BBH) mergers, but the detailed processes that lead to a BBH merger in an AGN disk are not yet well-constrained. We expect that binary formation in AGN disks is extremely efficient, and that initial binary hardening will occur due to gas drag. However, the gas drag will eventually become inefficient, leading to binaries that stall at semi-major axes too large for a gravitational wave-driven merger to occur in less than the AGN disk lifetime. Thus, dynamical encounters could play a critical role in hardening (or disrupting) binaries in the AGN channel. The imprint of dynamical processes particular to AGN disks could appear in the gravitational wave signal of such BBH mergers.

Most dynamical channels for BBH mergers are rotationally symmetric and therefore predict a symmetric distribution of effective spin around zero. However, there are several natural sources of symmetry breaking in the AGN channel for BBH mergers. We show via numerical experiments with the high-accuracy, high-precision few-body code that broken symmetry in dynamical encounters in AGN disks can lead to an asymmetry between prograde and retrograde BBH mergers. Under the assumption that the spin of the BHs becomes aligned with the angular momentum of the disk on a short timescale compared with the encounter timescale, an asymmetric distribution of effective spin is predicted in LIGO-Virgo detections of BBH mergers from AGN disks.