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

Yi-Han Wang,Rosalba Perna,Saavik Ford,Mordecai-Mark Mac Low,Barry Mckernan,Nathan W C Leigh

doi:10.3847/2041-8213/ac400a

Abstract

Abstract The disks of active galactic nuclei (AGNs) may be important sites of binary black hole (BBH) mergers. Here we show via numerical experiments with the high-accuracy, high-precision code SpaceHub that broken symmetry in dynamical encounters in AGN disks can lead to asymmetry between prograde and retrograde BBH mergers. The direction of the hardening asymmetry depends on the initial binary semimajor axis. 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 mass-weighted projected spin χ eff is predicted in LIGO–Virgo detections of BBH mergers from AGN disks. In particular, this model predicts that positive χ eff BBH mergers are most likely for encounters with massive tertiaries in migration traps at radial distances ≳500–600 gravitational radii.

Highlights

IntroductionActive galactic nucleus (AGN) disks may be important sites for stellar mass binary black hole (BBH) mergers (McKernan et al 2012; Bartos et al 2017; Stone et al 2017; Leigh et al 2018; Samsing et al 2020), but the detailed processes that lead to a BBH merger in an AGN disk are not yet well-constrained (e.g. McKernan et al 2018; Grobner et al 2020)
We show via numerical experiments with the high-accuracy, high precision code SpaceHub that broken symmetry in dynamical encounters in Active galactic nucleus (AGN) disks can lead to an asymmetry between prograde and retrograde binary black hole (BBH) mergers
Active galactic nucleus (AGN) disks may be important sites for stellar mass binary black hole (BBH) mergers (McKernan et al 2012; Bartos et al 2017; Stone et al 2017; Leigh et al 2018; Samsing et al 2020), but the detailed processes that lead to a BBH merger in an AGN disk are not yet well-constrained (e.g. McKernan et al 2018; Grobner et al 2020)

Summary

Introduction

Active galactic nucleus (AGN) disks may be important sites for stellar mass binary black hole (BBH) mergers (McKernan et al 2012; Bartos et al 2017; Stone et al 2017; Leigh et al 2018; Samsing et al 2020), but the detailed processes that lead to a BBH merger in an AGN disk are not yet well-constrained (e.g. McKernan et al 2018; Grobner et al 2020). We expect that binary formation in AGN disks is extremely efficient (Secunda et al 2020; Tagawa et al 2020a), and that initial binary hardening will occur due to gas drag We expect gas drag will eventually become inefficient (e.g. Leigh et al 2014), 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. Dynamical encounters could play a critical role in hardening (or disrupting) binaries in the AGN channel Most dynamical channels for BBH mergers are rotationally symmetric and predict a symmetric distribution of χeff around χeff = 0 (e.g. Rodriguez et al 2018; Liu & Lai 2017). Hereafter we denote prograde and retrograde binaries as (+) or (−) respectively and prograde/retrograde tertiaries as +/−

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Conclusion