Massive black hole remnants of the first stars – III. Observational signatures from the past

The first stars forming in minihaloes at redshifts z > 20 may have been very massive and could have left behind massive black hole (MBH) remnants. In previous papers we investigated the hierarchical merging of these ‘seed’ 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. We also estimated accretion luminosities for these MBHs and found them to be consistent with observations of ultraluminous X‐ray point sources. Here we compute the strength of gravitational wave events as MBHs merge to form the more massive black holes that we predict reside in galaxy haloes today. If MBHs merge efficiently, we predict that as many as 10 4 –10 5 events per year may fall within the sensitivity limits of the proposed Laser Interferometer Space Antenna gravitational wave observatory. The collapse of the first massive stars to form MBHs may also be accompanied by gamma‐ray bursts (GRBs). If this is the case and if GRBs are observable out to the redshifts of first star formation, we predict that about 10 5 –10 6 GRBs per year could be detected. As merging MBH binaries reach their last stable orbits before final coalescence, a fraction of the gravitational wave energy may be released as a pulse of gamma‐rays (for instance, through interaction with material enveloping a merging MBH binary). This fraction has to be larger than about 10 −2 for MBH mergers to account for some beamed GRBs, and greater than 10 −6 for the gamma‐rays to be detectable out to cosmological distances with upcoming GRB detector missions.