Intermediate-Mass Ratio Inspirals in Galactic Nuclei

TL;DR

This study investigates the dynamical evolution and merger rates of intermediate-mass and stellar-mass black hole binaries near supermassive black holes in galactic nuclei, highlighting their potential as gravitational wave sources.

Contribution

It introduces detailed N-body simulations including Post-Newtonian effects to analyze the formation and merger of IMBH-SBH binaries in galactic centers, emphasizing the role of Kozai-Lidov oscillations.

Findings

Mass segregation and Kozai-Lidov oscillations accelerate mergers.

Estimated merger rate is approximately 1 Gpc^{-3} yr^{-1}.

Observable signatures include tidal debris and potential tidal disruption events.

Abstract

In this paper, we study the secular dynamical evolution of binaries composed of intermediate-mass and stellar-mass black holes (IMBHs and SBHs, respectively) in orbit about a central super-massive black hole (SMBH) in galactic nuclei. Such BH triplets could form via the inspiral of globular clusters toward galactic nuclei due to dynamical friction, or even major/minor galaxy mergers. We perform, for reasonable initial conditions that we justify, sophisticated $N$-body simulations that include both regularization and Post-Newtonian corrections. We find that mass segregation combined with Kozai-Lidov oscillations induced by the primary SMBH can effectively merge IMBH-SBH binaries on time-scales much shorter than gravitational wave emission alone. Moreover, the rate of such extreme mass ratio inspirals could be high ($\sim 1\ \mathrm{Gpc}^{-3}\ \mathrm{yr}^{-1}$) in the local Universe, but these are expected to be associated with recent GC infall or major/minor mergers, making the observational signatures of such events (e.g., tidal debris) good diagnostics for searching for SMBH-IMBH-SBH mergers. A small fraction could also be associated with tidal disruption events by the IMBH-SBH during inspiral.