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Supermassive black holes with billion solar masses are in place alreadywithin the first Gyr, however, their origin and growth in such a short lapse oftime is extremely challenging to understand. Here, we discuss the formationpaths of early black-hole seeds, showing the limits of light black-hole seedsfrom stellar origin and the expected characteristics of heavy/massiveblack-hole seeds originated by gas direct collapse in peculiar primordialconditions. To draw conclusions on the possible candidates and the role of theambient medium, we use results from N-body hydrodynamic simulations includingatomic and molecular non-equilibrium abundance calculations, cooling, starformation, feedback mechanisms, stellar evolution, metal spreading of severalheavy elements from SNII, AGB and SNIa, and multifrequency radiative transferover 150 frequencies coupled to chemistry and SED emission for popII-I andpopIII stellar sources. Standard stellar-origin light black holes are unlikelyto be reliable seeds of early supermassive black holes, because, underrealistic assumptions, they cannot grow significantly in less than a billionyears. Alternatively, massive black-hole seeds might originate from directcollapse of pristine gas in primordial quiescent mini-haloes that are exposedto stellar radiation from nearby star forming regions. The necessary conditionsrequired to form these heavy seeds must be complemented with information on thecomplex features of local environments and the fine balance between chemistryevolution and radiative transfer.

Early black-hole seeds in the first billion years