Chasing the observational signatures of seed black holes at z > 7:
  candidate observability
  Observing the light emitted by the first accreting black holes (BHs) would
dramatically improve our understanding of the formation of quasars at z > 6,
possibly unveiling the nature of their supermassive black hole (SMBH) seeds. In
previous works we explored the relative role of the two main competing BH seed
formation channels, Population III remnants (low-mass seeds) and direct
collapse BHs (high-mass seeds), investigating the properties of their host
galaxies in a cosmological context. Building on this analysis, we predict here
the spectral energy distribution and observational features of low- and
high-mass BH seeds selected among the progenitors of a z~6 SMBH. We derive the
processed emission from both accreting BHs and stars by using the
photo-ionization code Cloudy, accounting for the evolution of metallicity and
dust-to-gas mass ratio in the interstellar medium of the host galaxies, as
predicted by the cosmological data- constrained model GAMETE/QSOdust. We show
how future missions like JWST and ATHENA will be able to detect the light
coming from SMBH progenitors already at z~16. We build upon previous
complementary studies and propose a method based on the combined analysis of
near infrared (NIR) colors, IR excess (IRX) and UV continuum slopes (i.e.
color-color and IRX-Beta diagrams) to distinguish growing seed BH host galaxies
from starburst-dominated systems in JWST surveys. Sources selected through this
criterion would be the best target for follow-up X-ray observations.
