Light, medium-weight, or heavy? The nature of the first supermassive black hole seeds
Federica Sassano, Raffaella Schneider, Rosa Valiante, Kohei Inayoshi, Sunmyon Chon, Kazuyuki Omukai, Lucio Mayer, Pedro R. Capelo
Abstract
ABSTRACT Observations of hyper-luminous quasars at z>6 reveal the rapid growth of supermassive black holes (SMBHs ${\gt}10^9 \,\rm M_{\odot }$) 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 (${\lt}10^3\, \rm M_\odot$) form at a higher rate, the mass growth of a ${\sim}10^9\, \rm M_\odot$ SMBH at z<15 is driven by efficient gas accretion (at a sub-Eddington rate) on to its heavy progenitors ($10^5\, \rm M_\odot$). 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 (${\lt} 10{-}20{{\ \rm per\ cent}}$) of all the BHs that seed galaxies at z>15.