Formation of massive black holes in ultra-compact dwarf galaxies:
  migration of primordial intermediate-mass black holes in N-body simulation
  Recent observational studies of ultra-compact dwarf galaxies (UCDs) have
discovered massive black holes (MBHs) with masses of more than ${10^6~\rm
M_\odot}$, in their central regions. We here consider that these MBHs can be
formed through merging of intermediate-mass black holes (IMBH), with masses of
${[10^3-10^5]~{\rm M}_{\odot}}$, within the stellar nuclei of dwarf galaxies,
which are progenitors of UCDs. We numerically investigate this formation
process for a wide range of model parameters using N-body simulations. This
means that IMBH growth and feedback is neglected in this study. We find that
only massive IMBHs of $10^5~\rm M_\odot$ sink into the central regions of their
host dwarf ($\approx 10^{10}~\rm M_\odot$) to be gravitationally trapped by its
stellar nucleus within less than 1 Gyr in most dwarf models. We also find that
lighter IMBHs with $[1 - 30] \times 10^3~\rm M_\odot$ sink into the centre in
low-mass dwarfs ($\approx 10^{9}~\rm M_\odot$) due to more efficient dynamical
friction (DF). Additionally, we show that the IMBHs can form binaries in the
centre and, rarely, before they reach the centre, which may lead to the IMBHs
merging and thus emitting gravitational waves that could be detected by LISA.
Finally, we discuss the required number of IMBHs for the MBH formation in UCDs
and the physical roles of stellar nuclei in IMBH binaries and mergers.
