  We investigate the possibility that present-day galaxies and their dark
matter haloes contain a population of massive black holes (MBHs) that form by
hierarchical merging of the black hole remnants of the first stars in the
Universe. Some of the MBHs may be large enough or close enough to the centre of
the galactic host that they merge within a Hubble time. We estimate to what
extent this process could contribute to the mass of the super-massive black
holes (SMBHs) observed in galactic centres today. The relation between SMBH and
galactic bulge mass in our model displays the same slope as that found in
observations. Many MBHs will not reach the centre of the host halo, however,
but continue to orbit within it. In doing so MBHs may remain associated with
remnants of the satellite halo systems of which they were previously a part.
Using a semi-analytical approach that explicitly accounts for dynamical
friction, tidal disruption and encounters with galactic disks, we follow the
hierarchical merging of MBH systems and their subsequent dynamical evolution
inside the respective host haloes. We predict the mass and abundance of MBHs in
present-day galactic haloes, and also estimate the MBH mass accretion rates as
well as bolometric luminosities for two different accretion scenarios. MBHs
that have not undergone recent merging will retain associated dark matter cusps
that were enhanced by black hole accretion growth, and may be possible sources
of gamma rays via neutralino annihilations.
