2021MNRAS.506..613S

Observations of hyper-luminous quasars at z>6 reveal the rapid growth of supermassive black holes (SMBHs >10⁹M_☉) whose origin is still difficult to explain. Their progenitors may have formed as remnants of massive, metal-free stars (light seeds), via stellar collisions (medium-weight seeds) and/or massive gas clouds direct collapse (heavy seeds). In this work, we investigate for the first time the relative role of these three seed populations in the formation of z>6 SMBHs within an Eddington-limited gas accretion scenario. To this aim, we implement in our semi-analytical data-constrained model a statistical description of the spatial fluctuations of Lyman-Werner (LW) photodissociating radiation and of metal/dust enrichment. This allows us to set the physical conditions for black hole seeds formation, exploring their relative birth rate in a highly biased region of the Universe at z>6. We find that the inclusion of medium-weight seeds does not qualitatively change the growth history of the first SMBHs: although less massive seeds (<10³M_☉) form at a higher rate, the mass growth of a ∼10⁹M_☉ SMBH at z<15 is driven by efficient gas accretion (at a sub-Eddington rate) on to its heavy progenitors (10⁵M_☉). This conclusion holds independently of the critical level of LW radiation and even when medium-weight seeds are allowed to form in higher metallicity galaxies, via the so-called supercompetitive accretion scenario. Our study suggests that the genealogy of z∼6 SMBHs is characterized by a rich variety of BH progenitors, which represent only a small fraction (< 10-20 per cent) of all the BHs that seed galaxies at z>15.