Accelerated orbital decay of supermassive black hole binaries in merging
  nuclear star clusters
  The coalescence of supermassive black holes (SMBHs) should generate the
strongest sources of gravitational waves (GWs) in the Universe. However, the
dynamics of their coalescence is the subject of much debate. In this study, we
use a suite of $N$-body simulations to follow the merger of two nuclear star
clusters (NSCs), each hosting a SMBH in their centre. We find that the presence
of distinct star clusters around each SMBH has important consequences for the
dynamical evolution of the SMBH binary: (i) The separation between the SMBHs
decreases by a few orders of magnitude in the first few Myrs by the combined
effects of dynamical friction and a drag force caused by tidally stripped
stars. In fact, this is a significant speedup for equal mass ratio binaries,
and becomes extreme for unequal mass ratios, e.g. 1:10 or 1:100, which
traditional dynamical friction alone would not permit to bind. (ii) The
subsequent binary hardening is driven by the gravitational slingshots between
the SMBH binary and stars, and also depends on the mass ratio between the
SMBHs. Thus, with this additional drag force, we find that all SMBHs in our
suite coalesce within a Hubble time. Given that about 50% of Milky Way sized
galaxies host NSCs, our results are encouraging for upcoming GW observations
with the Laser Interferometer Space Antenna -- LISA -- which will detect SMBH
coalescence in the $10^4-10^7$$M_{\rm \odot}$ mass range.
