  Motivated by the recent detection of gravitational waves from the black hole
binary merger GW150914, we study the dynamical evolution of black holes in
galactic nuclei where massive star clusters reside. With masses of ~10^7M_Sun
and sizes of only a few parsecs, nuclear star clusters are the densest stellar
systems observed in the local universe and represent a robust environment where
(stellar mass) black hole binaries can dynamically form, harden and merge. We
show that due to their large escape speeds, nuclear star clusters can keep a
large fraction of their merger remnants while also evolving rapidly enough that
the holes can sink back to the central regions where they can swap in new
binaries that can subsequently harden and merge. This process can repeat
several times and produce black hole mergers of several tens of solar masses
similar to GW150914 and up to a few hundreds of solar masses, without the need
of invoking extremely low metallicity environments or implausible initial
conditions. We use a semi-analytical approach to describe the formation and
dynamics of black holes in massive star clusters. We find a black hole binary
merger rate per volume from nuclear star clusters of ~1.5 Gpc^-3 yr^-1,
implying up to a few tens of possible detections per year with Advanced LIGO.
Our models suggest a local merger rate of 0.3- 1 Gpc^-3 yr^-1 for high mass
black hole binaries similar to GW150914 (total mass >~ 50 M_Sun, redshift z<
0.3); a merger rate comparable to that of high mass black hole binaries that
are dynamically assembled in globular clusters. Finally, we show that if all
black holes receive high natal kicks, >~50km s^-1, then nuclear star clusters
could dominate the local merger rate of binary black holes compared to the
merger rate of similar binaries produced in either globular clusters or through
isolated binary evolution.
