AGN Disks Harden the Mass Distribution of Stellar-mass Binary Black Hole Mergers

Abstract

The growing number of stellar-mass binary black hole mergers discovered by Advanced LIGO and Advanced Virgo are starting to constrain the binaries' origin and environment. However, we still lack sufficiently accurate modeling of binary formation channels to obtain strong constraints, or to identify sub-populations. One promising formation mechanism that could result in different black hole properties is binaries merging within the accretion disks of Active Galactic Nuclei (AGN). Here we show that the black holes' orbital alignment with the AGN disks preferentially selects heavier black holes. We carry out Monte Carlo simulations of orbital alignment with AGN disks, and find that AGNs harden the initial black hole mass function. Assuming an initial power law mass distribution M −β bh , we find that the power law index changes by ∆β ∼ 1.3, resulting in a more top-heavy population of merging black holes. This change is independent of the mass of, and accretion rate onto, the supermassive black hole in the center of the AGN. Our simulations predict an AGN-assisted merger rate of ∼ 4 Gpc −3 yr −1 . With its hardened mass spectra, the AGN channel could be responsible for 10 − 50% of gravitational-wave detections.

Hierarchical Black Hole Mergers in Active Galactic Nuclei

“…We calculated the Bayesian odds ratio P (AGN-GW170729)/P (obs.-GW170729). While the parameters of GW170729 are 5 times more likely to arise from our hierarhical-AGN distribution than from that of the null-hypothesis, taking into account a prior probability ratio P (AGN)/P (obs/) = 0.1 − 0.4 (Yang et al 2019), we find that the odds ratio is ∼ 1. More, similar events will be needed to probe this channel with high significance.…”

Properties and Astrophysical Implications of the 150 M⊙ Binary Black Hole Merger GW190521

“…As a consequence of the joint contribution of gas accretion, BBH mergers, and high escape velocities, a fraction of BHs in AGN disks are expected to have masses in the PI mass gap or even in the IMBH regime (McKernan et al 2012). Recent work suggests that they have a mass function similar to a power law, but significantly flatter than field BHs (Yang et al 2019b), or even reminiscent of a broken power law (McKernan et al 2018;Yang et al 2019a). The possible mass ratios are highly uncertain, though most mergers have mass ratios less extreme than 10:1 (McKernan et al 2018;Yang et al 2019a).…”

Fingerprints of Binary Black Hole Formation Channels Encoded in the Mass and Spin of Merger Remnants

“…The tool does not take into account the metallicity gradient in modeling BBH galaxy hosts, although this issue is partly addressed via defining different metallicity distribution choices (see, for instance, models ID 2a, b, and c). Our analysis excludes BBH mergers forming via alternative processes, like BBH formation around an SMBH or in an AGN disk (McKernan et al 2012(McKernan et al , 2014(McKernan et al , 2018Bartos et al 2017;Yang et al 2019), or in triples (Antonini & Perets 2012;Fragione et al 2019;Hoang et al 2018;Arca Sedda 2020). Moreover, our tool does not account for chemically homogeneous binary evolution (Marchant et al 2016).…”

“…These second generation BHs can significantly affect the BH mass spectrum. In galactic nuclei, BBH evolution and coalescence is even more complex due to the possible presence of a quiescent SMBH (Antonini & Perets 2012;Arca Sedda & Gualandris 2018;Hoang et al 2018;Arca Sedda & Capuzzo-Dolcetta 2019;Fernández & Kobayashi 2019;Hoang et al 2019;Rasskazov & Kocsis 2019) or an AGN in the galactic center (Bartos et al 2017;Yang et al 2019).…”