On the transition from nuclear-cluster to black-hole dominated galaxy
  cores
  Giant elliptical galaxies, believed to be built from the merger of lesser
galaxies, are known to house a massive black hole at their center rather than a
compact star cluster. If low- and intermediate-mass galaxies do indeed partake
in the hierarchical merger scenario, then one needs to explain why their dense
nuclear star clusters are not preserved in merger events. A valuable clue may
the recent revelation that nuclear star clusters and massive black holes
frequently co-exist in intermediate mass bulges and elliptical galaxies. In an
effort to understand the physical mechanism responsible for the disappearance
of nuclear star clusters, we have numerically investigated the evolution of
merging star clusters with seed black holes. Using black holes that are 1-5% of
their host nuclear cluster mass, we reveal how their binary coalescence during
a merger dynamically heats the newly wed star cluster, expanding it,
significantly lowering its central stellar density, and thus making it
susceptible to tidal destruction during galaxy merging. Moreover, this
mechanism provides a pathway to explain the observed reduction in the
nucleus-to-galaxy stellar mass ratio as one proceeds from dwarf to giant
elliptical galaxies.
