Early black-hole seeds in the first billion years
  Supermassive black holes with billion solar masses are in place already
within the first Gyr, however, their origin and growth in such a short lapse of
time is extremely challenging to understand. Here, we discuss the formation
paths of early black-hole seeds, showing the limits of light black-hole seeds
from stellar origin and the expected characteristics of heavy/massive
black-hole seeds originated by gas direct collapse in peculiar primordial
conditions. To draw conclusions on the possible candidates and the role of the
ambient medium, we use results from N-body hydrodynamic simulations including
atomic and molecular non-equilibrium abundance calculations, cooling, star
formation, feedback mechanisms, stellar evolution, metal spreading of several
heavy elements from SNII, AGB and SNIa, and multifrequency radiative transfer
over 150 frequencies coupled to chemistry and SED emission for popII-I and
popIII stellar sources. Standard stellar-origin light black holes are unlikely
to be reliable seeds of early supermassive black holes, because, under
realistic assumptions, they cannot grow significantly in less than a billion
years. Alternatively, massive black-hole seeds might originate from direct
collapse of pristine gas in primordial quiescent mini-haloes that are exposed
to stellar radiation from nearby star forming regions. The necessary conditions
required to form these heavy seeds must be complemented with information on the
complex features of local environments and the fine balance between chemistry
evolution and radiative transfer.
