Supermassive Seeds for Supermassive Black Holes

Recent observations of quasars powered by supermassive black holes (SMBHs) out to z > 7 constrain both the initial seed masses and the growth of the most massive black holes (BHs) in the early universe. Here we elucidate the implications of the radiative feedback from early generations of stars and from BH accretion for popular models for the formation and growth of seed BHs. We show that by properly accounting for (1) the limited role of mergers in growing seed BHs as inferred from cosmological simulations of early star formation and radiative feedback, (2) the sub-Eddington accretion rates of BHs expected at the earliest times, and (3) the large radiative efficiencies (eᵣad) of the most massive BHs inferred from observations of active galactic nuclei at high redshift (eᵣad > 0. 1), we are led to the conclusion that the initial BH seeds may have been as massive as > 10⁵ MSun. This presents a strong challenge to the Population III seed model, which calls for seed masses of ~ 100 MSun and, even with constant Eddington-limited accretion, requires eᵣad < 0. 09 to explain the highest-z SMBHs in today's standard LambdaCDM cosmological model. It is, however, consistent with the prediction of the direct collapse scenario of SMBH seed formation, in which a supermassive primordial star forms in a region of the universe with a high molecule-dissociating background radiation field, and collapses directly into a 10⁴--10⁶ MSun seed BH. These results corroborate recent cosmological simulations and observational campaigns which suggest that these massive BHs were the seeds of a large fraction of the SMBHs residing in the centers of galaxies today.