Black Hole Mergers in Galactic Nuclei Induced by the Eccentric Kozai-Lidov Effect
Abstract
Nuclear star clusters around a central massive black hole (MBH) are expected to be abundant in stellar black hole (BH) remnants and BH-BH binaries. These binaries form a hierarchical triple system with the central MBH, and gravitational perturbations from the MBH can cause high-eccentricity excitation in the BH-BH binary orbit. During this process, the eccentricity may approach unity, and the pericenter distance may become sufficiently small so that gravitational-wave emission drives the BH-BH binary to merge. In this work, we construct a simple proof-of-concept model for this process, and specifically, we study the eccentric Kozai-Lidov mechanism in unequal-mass, soft BH-BH binaries. Our model is based on a set of Monte Carlo simulations for BH-BH binaries in galactic nuclei, taking into account quadrupole- and octupole-level secular perturbations, general relativistic precession, and gravitational-wave emission. For a typical steady-state number of BH-BH binaries, our model predicts a total merger rate of ∼1-3 {Gpc} -3 {yr} -1, depending on the assumed density profile in the nucleus. Thus, our mechanism could potentially compete with other dynamical formation processes for merging BH-BH binaries, such as the interactions of stellar BHs in globular clusters or in nuclear star clusters without an MBH.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 2018
- DOI:
- 10.3847/1538-4357/aaafce
- arXiv:
- arXiv:1706.09896
- Bibcode:
- 2018ApJ...856..140H
- Keywords:
-
- black hole physics;
- galaxies: star clusters: general;
- gravitational waves;
- stars: kinematics and dynamics;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 10 pages, 12 figures, accepted to ApJ