Extremely efficient mergers of intermediate mass black hole binaries in
  nucleated dwarf galaxies
  Gravitational waves emitted by merging black holes between $\sim
10^4-10^7~M_\odot$ 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 ($10^7$~M$_{\odot}$ - $10^{10}$~M$_{\odot}$) binary
formation, coalescence and merger within massive galaxies is well-studied.
However, low-to-intermediate mass black holes (IMBHs) are hosted by low-mass
and dwarf galaxies; it is not trivial to extrapolate black hole merger
timescales to this IMBH binary regime, due to the starkly different host galaxy
structure, kinematics, and morphology compared to massive galaxy hosts. We
perform ultra-high 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 parsecs, 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 ($e \sim 0.4-0.99$) 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
predominantly 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 is 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.
