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 (e_rad) of the
most massive BHs inferred from observations of active galactic nuclei at high
redshift (e_rad > 0.1), we are led to the conclusion that the initial BH seeds
may have been as massive as > 10^5 M_Sun. This presents a strong challenge to
the Population III seed model, which calls for seed masses of ~ 100 M_Sun and,
even with constant Eddington-limited accretion, requires e_rad < 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 10^4--10^6 M_Sun 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.
