Massive black hole remnants of the first stars II: optical and X-ray signatures in present-day galactic haloes

Abstract

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.