Massive Black Holes as Population III Remnants

Recent numerical simulations of the fragmentation of primordial molecular clouds in hierarchical cosmogonies have suggested that the very first stars (the so-called Population III) may have been rather massive. Here we point out that a numerous population of massive black holes (MBHs) -- with masses intermediate between those of stellar and supermassive holes -- may be the endproduct of such an episode of pregalactic star formation. If only one MBH with m > 150 msun formed in each of the `minihalos' collapsing at z=20 from 3-sigma fluctuations, then the mass density of Pop III MBHs would be comparable to that of the supermassive variety observed in the nuclei of galaxies. Since they form in high-sigma rare density peaks, relic MBHs are predicted to cluster in the bulges of present-day galaxies as they become incorporated through a series of mergers into larger and larger systems. Dynamical friction would cause more than 50 (m/150 msun)^{1/2} such objects to sink towards the center. The presence of a small cluster of MBHs in galaxy nuclei may have several interesting consequences associated with tidal captures of ordinary stars (likely followed by disruption), MBH capture by the central supermassive black hole, gravitational wave radiation from such coalescences. Accreting pregalactic MBHs may be detectable as ultra-luminous, off-nuclear X-ray sources.