Resolving the Complex Evolution of a Supermassive Black Hole Triplet in
  a Cosmological Simulation
  We present here a self-consistent cosmological zoom-in simulation of a triple
supermassive black hole (SMBH) system forming in a complex multiple galaxy
merger. The simulation is run with an updated version of our code KETJU, which
is able to follow the motion of SMBHs down to separations of tens of
Schwarzschild radii while simultaneously modeling the large-scale astrophysical
processes in the surrounding galaxies, such as gas cooling, star formation, and
stellar and AGN feedback. Our simulation produces initially a SMBH binary
system for which the hardening process is interrupted by the late arrival of a
third SMBH. The KETJU code is able to accurately model the complex behavior
occurring in such a triple SMBH system, including the ejection of one SMBH to a
kiloparsec-scale orbit in the galaxy due to strong three-body interactions as
well as Lidov-Kozai oscillations suppressed by relativistic precession when the
SMBHs are in a hierarchical configuration. One pair of SMBHs merges $\sim
3\,\mathrm{Gyr}$ after the initial galaxy merger, while the remaining binary is
at a parsec-scale separation when the simulation ends at redshift $z=0$. We
also show that KETJU can capture the effects of the SMBH binaries and triplets
on the surrounding stellar population, which can affect the binary merger
timescales as the stellar density in the system evolves. Our results
demonstrate the importance of dynamically resolving the complex behavior of
multiple SMBHs in galactic mergers, as such systems cannot be readily modeled
using simple orbit-averaged semi-analytic models.
