  The first stars forming in minihaloes at redshifts greater than 20 may have
been very massive and could have left behind massive black hole (MBH) remnants.
In a previous paper we investigated the hierarchical merging of these MBHs and
their associated haloes, using a semi-analytical approach consisting of a
hierarchical merger tree algorithm and explicit prescriptions for the dynamics
of merged substructure inside a larger host halo following a merger. One of the
results was the prediction of a number of MBHs orbiting throughout present-day
galactic haloes. In addition we determined the mass accretion rate of these
MBHs assuming that they could in principle remain embedded in a gaseous core
that is a remnant of the haloes with which the MBHs were associated before
merging with the host system. On this basis we determine in this paper the
bolometric, optical and X-ray luminosity functions for the accreting MBHs using
thin disk and advection dominated accretion flow models. Our predicted MBH
X-ray emissions are then compared with observations of ultra-luminous X-ray
sources in galaxies. We find that the slope and normalisation of the predicted
X-ray luminosity function are consistent with the observations.
