Formation and Evolution of Compact Object Binaries in AGN Disks

Abstract

The astrophysical origin of gravitational wave (GW) events discovered by LIGO/VIRGO remains an outstanding puzzle. In active galactic nuclei (AGN), compact-object binaries form, evolve, and interact with a dense star cluster and a gas disk. While most binaries are soft and get disrupted by binary-single interactions, an important question is whether and how binaries can merge in these environments. To address this question, we have performed one-dimensional N -body simulations combined with a semi-analytical model which includes the formation, disruption, and evolution of binaries self-consistently. We point out that binaries can form in single-single interactions by the dissipation of kinetic energy in a gaseous medium. This "gas capture" binary formation channel contributes up to 97 % of gas-driven mergers and leads to a high merger rate in AGN disks even without pre-existing binaries. We find the merger rate to be in the range ∼ 0.02 − 60 Gpc −3 yr −1 , whose high end corresponds to top heavy initial mass functions, highly anisotropic initial BH distribution, short AGN lifetime, and large AGN disk sizes. The results are insensitive to the assumptions on gaseous hardening processes: we find that once they are formed, binaries merge efficiently via binary-single interactions even if these gaseous processes are neglected. We find that the average number of mergers per BH is 0.4, and the probability for repeated mergers in 30 Myr is ∼ 0.21 − 0.45. High BH masses due to repeated mergers, high eccentricities, and a significant Doppler drift of GWs are promising signatures which distinguish this merger channel from others. Furthermore, we find that gas-capture binaries reproduce the distribution of LMXBs in the Galactic center, including an outer cutoff at ∼ 1 pc due to the competition between migration and hardening by gas torques.

Hierarchical mergers of stellar-mass black holes and their gravitational-wave signatures

“…For the formation channels highlighted below, current BH-binary merger-rate predictions range from 4 − 60 Gpc −3 yr −1 (at z = 0) for globular clusters, 97,98 ∼ 1 Gpc −3 yr −1 for nuclear star clusters, 99,100 and 0.002 − 60 Gpc −3 yr −1 for AGN disks. 101,102 It is worth noting that orbital eccentricities also affect merger recoils. For eccentricities 0.3, BH kicks can be enhanced by up to 25%, [103][104][105] making it more difficult to retain the remnants of mergers formed in dynamical environments, a fraction of which is predicted to be eccentric.…”

“…Gaseous AGN disks are also promising environments for the production of BH binaries merging in the LIGO/Virgo band. 21,101,102,[158][159][160][161] In this scenario, stellar-mass BHs are embedded in accretion disks surrounding supermassive BHs, and their evolution is driven by angular-momentum transfer via viscous interactions -a process that is analogous to that of planetary While all channels naturally produce 1g BHs (blue) either from stellar collapse or cosmological perturbations, only a subset of them can efficiently assemble hierarchical mergers (orange). Solid (dashed) lines indicate formation channels that are efficient (inefficient) at producing hierarchical mergers, though their overall contribution to the observed population will also depend on the relative merger rates from the different channels.…”

“…Hierarchical mergers in AGN disks have been explored, with focus on both intermediate mass 166,167 and stellar-mass events detectable by LIGO/Virgo. 102,160,168,169 Assuming that (i) the number of BHs dragged into a migration trap within the AGN lifetime follows a Poisson distribution and (ii) once a new BH reaches the trap it mergers immediately with the BH that is already there, Yang et al 169 found that the fraction of highergeneration mergers from AGN disks is 50%. Combining N-body simulations and analytical arguments, Tagawa et al 102 reports fractions between ∼ 20% and ∼ 45%, although in their models migration traps are less relevant to the overall merger rate.…”

The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei

“…Over the past several years LIGO/Virgo has detected dozens of BBHs mergers (Abbott et al 2019;The LIGO Scientific Collaboration et al 2020;Abbott et al 2020a) which are typically attributed to formation through isolated binary evolution (Belczynski et al 2016;Giacobbo & Mapelli 2018;Kruckow et al 2018;Schrøder et al 2018;Bavera et al 2020) or dynamical interactions in dense stellar systems (O'Leary et al 2009;Antonini & Perets 2012;Samsing et al 2014;Antonini et al 2017;Askar et al 2017;Samsing & Ramirez-Ruiz 2017;Banerjee 2018;Fragione & Kocsis 2018;Rodriguez et al 2018;Samsing & D'Orazio 2018;Kremer et al 2018Kremer et al , 2019Di Carlo et al 2019). However, a potentially significant fraction of all LIGO/Virgo events may be due to BBHs that merge within an AGN disk (McKernan et al 2012(McKernan et al , 2014Bartos et al 2017;Stone et al 2017;Tagawa et al 2019). Due to the high gas densities within the disk, accretion and gas drag will help the binary inspiral much faster than via three-body hardening and gravitational wave emission alone (Antoni et al 2019).…”

“…Here, the gravitational influence of the AGN disk is strong, and the mass-segregated black holes will gradually have their inclinations and eccentricities damped until they occupy circular orbits embedded within the AGN disk (Ward & Hahn 1994;Tanaka & Ward 2004). Single, rather than binary, BHs may also become embedded by these same mechanisms and then efficiently pair up via gas-mediated single-single encounters (Tagawa et al 2019).…”