2021MNRAS.508.1174K

Gravitational waves emitted by merging black holes between ∼10⁴ and 10⁷ M_☉ will be detectable by the Laser Interferometer Space Antenna (LISA) with signal-to-noise ratios of several hundred out to redshift 20. Supermassive black hole (10⁷-10¹⁰ M_☉) binary formation, coalescence, and merger within massive galaxies is well-studied. However, intermediate-mass black holes (IMBHs) between ∼10⁴ and 10⁶ M_☉ are hosted by low mass and dwarf galaxies; it is not trivial to extrapolate black hole merger time-scales to this IMBH binary regime, due to the starkly different host galaxy structure, kinematics, and morphology compared to massive galaxy hosts. We perform ultrahigh resolution N-body simulations to study IMBH dynamics in nucleated dwarf galaxies whose structural parameters are obtained from observations of nearby dwarf galaxies. Starting from 50 pc, an IMBH quickly forms a binary. Thereafter, the binary orbit shrinks rapidly due to the high central stellar densities furnished by nuclear star clusters (NSCs). We find high eccentricities (e ∼ 0.4-0.99) in our suite of IMBH binaries, and residual eccentricity may persist to the LISA regime. IMBH merger times are typically a few hundred million years, with a few exceptionally short merger times for high eccentricities. We find that IMBH-stellar encounters originate pre-dominantly from NSCs, if the NSC-to-IMBH binary mass ratio is greater than 10; otherwise, bulge stars contribute significantly. As the IMBH binary ejects stars, however, the NSCs are disrupted. We conclude that comparable-mass IMBHs merge very efficiently in nucleated dwarf galaxies, making them promising LISA sources, as well as a channel for IMBH growth.