  We investigate the consequences of a model of the assembly and growth of
massive black holes from primordial seeds, remnants of the first generation of
stars in a hierarchical structure formation scenario. Our model traces the
build-up of MBHs from an early epoch, and follows the merger history of dark
matter halos and their associated holes via Monte Carlo realizations of the
merger hierarchy from early times to the present time. The sequence of minor
and major mergers experienced by galactic halos in their hierarchical growth
affects the merger history of MBHs embedded in their nuclei. So, if the
formation route for the assembly of SMBHs dates back to the early universe, a
large number of BH interactions is inevitable. Binary black holes coalescence
timescales can be long enough for a third BH to fall in and interact with the
central binary. These BH triple interactions lead typically to the final
expulsion of one of the three bodies and to the recoil of the binary. Also,
asymmetric emission of gravitational waves in the last stages of the black hole
merging can give a recoil velocity to the centre of mass of the coalescing
binary. This scenario leads to the prediction of a population of intermediate
mass BHs (IMBHs) wandering in galaxy halos at the present epoch. We compute the
luminosity distribution produced by these IMBHs accreting from their
circumstellar medium. We find that in a Milky Way-sized galaxy they are unable
to account for sources with luminosities > 1e39 erg/s unless they carry a
baryonic remnant from which they are able to accrete for a long time. We also
find that, for typical spiral galaxies, the bright end of the point source
distribution correlates with the mass of the galaxy, and the most luminous
sources are expected to be found in the disk.
