Massive Black Holes as Population III Remnants

Recent numerical simulations of the fragmentation of primordial molecularclouds in hierarchical cosmogonies have suggested that the very first stars(the so-called Population III) may have been rather massive. Here we point outthat a numerous population of massive black holes (MBHs) -- with massesintermediate between those of stellar and supermassive holes -- may be theendproduct of such an episode of pregalactic star formation. If only one MBHwith m > 150 msun formed in each of the `minihalos' collapsing at z=20 from3-sigma fluctuations, then the mass density of Pop III MBHs would be comparableto that of the supermassive variety observed in the nuclei of galaxies. Sincethey form in high-sigma rare density peaks, relic MBHs are predicted to clusterin the bulges of present-day galaxies as they become incorporated through aseries of mergers into larger and larger systems. Dynamical friction wouldcause 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 severalinteresting consequences associated with tidal captures of ordinary stars(likely followed by disruption), MBH capture by the central supermassive blackhole, gravitational wave radiation from such coalescences. Accretingpregalactic MBHs may be detectable as ultra-luminous, off-nuclear X-raysources.Comment: 5 pages, 1 figure, slightly revised version, accepted for publication in the ApJ