Multiple supermassive black hole systems: SKA's future leading role
  Galaxies and supermassive black holes (SMBHs) are believed to evolve through
a process of hierarchical merging and accretion. Through this paradigm,
multiple SMBH systems are expected to be relatively common in the Universe.
However, to date there are poor observational constraints on multiple SMBHs
systems with separations comparable to a SMBH gravitational sphere of influence
(<< 1 kpc). In this chapter, we discuss how deep continuum observations with
the SKA will make leading contributions towards understanding how multiple
black hole systems impact galaxy evolution. In addition, these observations
will provide constraints on and an understanding of stochastic gravitational
wave background detections in the pulsar timing array sensitivity band (nanoHz
-microHz). We also discuss how targets for pointed gravitational wave
experiments (that cannot be resolved by VLBI) could potentially be found using
the large-scale radio-jet morphology, which can be modulated by the presence of
a close-pair binary SMBH system. The combination of direct imaging at high
angular resolution; low-surface brightness radio-jet tracers; and pulsar timing
arrays will allow the SKA to trace black hole binary evolution from separations
of a galaxy virial radius down to the sub-parsec level. This large dynamic
range in binary SMBH separation will ensure that the SKA plays a leading role
in this observational frontier.
