Eccentric Black Hole Mergers in Active Galactic Nuclei

Hiromichi Tagawa,Johan Samsing,Imre Bartos,Kazuyuki Omukai,Zoltán Haiman,Bence Kocsis

doi:10.3847/2041-8213/abd4d3

Hiromichi Tagawa, Johan Samsing + Show 4 more

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https://doi.org/10.3847/2041-8213/abd4d3

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Abstract

Abstract The astrophysical origin of gravitational wave transients is a timely open question in the wake of discoveries by the Laser Interferometer Gravitational-Wave Observatory (LIGO)/Virgo. In active galactic nuclei (AGNs), binaries form and evolve efficiently by interaction with a dense population of stars and the gaseous AGN disk. Previous studies have shown that stellar-mass black hole (BH) mergers in such environments can explain the merger rate and the number of suspected hierarchical mergers observed by LIGO/Virgo. The binary eccentricity distribution can provide further information to distinguish between astrophysical models. Here we derive the eccentricity distribution of BH mergers in AGN disks. We find that eccentricity is mainly due to binary–single (BS) interactions, which lead to most BH mergers in AGN disks having a significant eccentricity at 0.01 Hz, detectable by the Laser Interferometer Space Antenna. If BS interactions occur in isotropic-3D directions, then 8%–30% of the mergers in AGN disks will have eccentricities at 10 Hz above e 10 Hz ≳ 0.03, detectable by LIGO/Virgo/Kamioka Gravitational Wave Detector, while 5%–17% of mergers have e 10 Hz ≥ 0.3. On the other hand, if BS interactions are confined to the AGN–disk plane due to torques from the disk, with 1–20 intermediate binary states during each interaction, or if BHs can migrate to ≲ 10−3 pc from the central supermassive BH, then 10%–70% of the mergers will be highly eccentric (e 10 Hz ≥ 0.3), consistent with the possible high eccentricity in GW190521.

Highlights

Recent detections of gravitational waves (GWs) have shown evidence for a high black hole (BH) merger rate in the Universe (Abbott et al 2019a)
In this Letter, we investigate the distribution of the binary eccentricity for mergers in active galactic nuclei (AGN) disks by performing one dimensional (1D) N -body simulations combined with semi-analytical prescriptions, which enable us to follow binaries considering such effects as eccentricity evolution due to BS interactions, type I/II torques exerted by circumbinary disks, and GW radiation (§ 2.3)
For M3, the χeff and χp distributions are not shown since χp is always 0 due to the assumption that the orbital planes of SS-GW capture (GWC) binaries are aligned with the AGN disk plane (§ 2.2.1)

Summary

Introduction

Recent detections of gravitational waves (GWs) have shown evidence for a high black hole (BH) merger rate in the Universe (Abbott et al 2019a). The proposed astrophysical pathways to merger remain debated. Measuring the binary eccentricity (e) is useful to distinguish between possible astrophysical pathways to merger. The feasibility to measure e has increased tremendously due to the improvement of the detectors and GW data analysis methods Nishizawa et al 2016; Lower et al.2018; Abbott et al 2019b; Romero-Shaw et al 2019). The LIGO, Virgo, and KAGRA detectors may detect eccentricities at 10 Hz above e10Hz 0.03. (Lower et al 2018; Gondan & Kocsis 2019; Romero-Shaw et al 2019), while LISA will detect e if above 10−3–.

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