  Observations suggest that star clusters often form in binaries or larger
bound groups. Therefore, mergers between two clusters are likely to occur. If
these clusters both harbor an intermediate-mass black hole (IMBH; 10^{2-4}
Msun) in their center, they can become a strong source of gravitational waves
when the black holes merge with each other. In order to understand the
dynamical processes that operate in such a scenario, one has to study the
evolution of the merger of two such young massive star clusters, and more
specifically, their respective IMBHs. We employ the direct-summation Nbody4
numerical tool on special-purpose GRAPE6 hardware to simulate a merger of two
stellar clusters each containing 63,000 particles and a central IMBH. This
allows us to study accurately the orbital evolution of the colliding clusters
and the embedded massive black holes. Within ~7 Myr the clusters have merged
and the IMBHs constitute a hard binary. The final coalescence happens in ~10^8
yrs. The implication of our analysis is that intermediate-mass black holes
merging as the result of coalescence of young dense clusters could provide a
source for the Laser Interferometer Space Antenna (LISA) space-based
gravitational wave detector mission. We find that interactions with stars
increase the eccentricity of the IMBH binary to about 0.8. Although the binary
later circularizes by emission of gravitational waves, the residual
eccentricity can be detectable through its influence on the phase of the waves
if the last few years of inspiral are observed.
