Fast coalescence of massive black hole binaries from mergers of galactic
  nuclei: implications for low-frequency gravitational-wave astrophysics
  We investigate a purely stellar dynamical solution to the Final Parsec
Problem. Galactic nuclei resulting from major mergers are not spherical, but
show some degree of triaxiality. With $N$-body simulations, we show that
massive black hole binaries (MBHB) hosted by them will continuously interact
with stars on centrophilic orbits and will thus inspiral---in much less than a
Hubble time---down to separations at which gravitational wave (GW) emission is
strong enough to drive them to coalescence. Such coalescences will be important
sources of GWs for future space-borne detectors such as the {\it Laser
Interferometer Space Antenna} (LISA). Based on our results, we expect that LISA
will see between $\sim 10$ to $\sim {\rm few} \times 10^2$ such events every
year, depending on the particular MBH seed model as obtained in recent studies
of merger trees of galaxy and MBH co-evolution. Orbital eccentricities in the
LISA band will be clearly distinguishable from zero with $e \gtrsim
0.001-0.01$.
