Intermediate-mass ratio inspirals in galactic nuclei

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 supermassive black hole (SMBH) in galactic nuclei. Such BH triplets could form via the inspiral of globular clusters (GC) 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 (∼1 Gpc^-3 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.