Gravitational waves from scattering of stellar-mass black holes in galactic nuclei

TL;DR

This paper models the evolution of stellar-mass black holes in galactic nuclei, predicts gravitational wave event rates, and highlights the distinctive eccentricity of mergers detectable by LIGO.

Contribution

It introduces a numerical approach to study black hole segregation, binary formation, and merger rates in galactic nuclei, emphasizing the significance of eccentric mergers for gravitational wave detection.

Findings

Black holes self-segregate around SMBHs.

Estimated BH coalescence rate: 1-100 per year.

Eccentric mergers detectable by LIGO up to 700 solar masses.

Abstract

Stellar mass black holes (BHs) are expected to segregate and form a steep density cusp around supermassive black holes (SMBHs) in galactic nuclei. We follow the evolution of a multi-mass system of BHs and stars by numerically integrating the Fokker-Planck energy diffusion equations for a variety of BH mass distributions. We find that the BHs "self-segregate'', and that the rarest, most massive BHs dominate the scattering rate closest to the SMBH (< 0.1 pc). BH--BH binaries form out of gravitational wave emission during BH encounters. We find that the expected rate of BH coalescence events detectable by Advanced LIGO is ~1 - 100/yr, depending on the initial mass function of stars in galactic nuclei and the mass of the most massive BHs. We find that the actual merger rate is likely ~10 times larger than this due to the intrinsic scatter of stellar densities in many different galaxies. The BH binaries that form this way in galactic nuclei have significant eccentricities as they enter the LIGO band (90% with e > 0.9), and are therefore distinguishable from other binaries, which circularise before becoming detectable. We also show that eccentric mergers can be detected to larger distances and greater BH masses than circular mergers, up to ~ 700 solar masses. Future ground-based gravitational wave observatories will be able to constrain both the mass function of BHs and stars in galactic nuclei.