The Ecology of the Galactic Centre: Nuclear Stellar Clusters and
  Supermassive Black Holes
  Supermassive black holes are found in most galactic nuclei. A large fraction
of these nuclei also contain a nuclear stellar cluster surrounding the black
hole. Here we consider the idea that the nuclear stellar cluster formed first
and that the supermassive black hole grew later. In particular we consider the
merger of three stellar clusters to form a nuclear stellar cluster, where some
of these clusters contain a single intermediate-mass black hole (IMBH). In the
cases where multiple clusters contain IMBHs, we discuss whether the black holes
are likely to merge and whether such mergers are likely to result in the
ejection of the merged black hole from the nuclear stellar cluster. In some
cases, no supermassive black hole will form as any merger product is not
retained. This is a natural pathway to explain those galactic nuclei that
contain a nuclear stellar cluster but apparently lack a supermassive black
hole; M33 being a nearby example. Alternatively, if an IMBH merger product is
retained within the nuclear stellar cluster, it may subsequently grow, e.g. via
the tidal disruption of stars, to form a supermassive black hole.
