Light, medium-weight, or heavy? The nature of the first supermassive black hole seeds

Abstract

Observations of hyper-luminous quasars at z > 6 reveal the rapid growth of supermassive black holes (SMBHs $>10^9 \rm M_{\odot }$) 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) photo-dissociating radiation and of metal/dust enrichment. This allows us to set the physical conditions for BH 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 ($<10^3 \rm M_\odot$) 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 super-competitive 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 ($< 10 - 20{{\ \rm per\ cent}}$) of all the BHs that seed galaxies at z > 15.

Astraeus VI: Hierarchical assembly of AGN and their large-scale effect during the Epoch of Reionization

“…However, since we are not directly interested in measuring merger rates in this work, we follow the results of Piana et al (2021b) who found that modelling delayed BH mergers had little impact on the actual growth of the SMBH. To be conservative, we also explored a model inspired by Sassano et al (2021) where BHs only merge rapidly during major mergers, assuming that the secondary BH in a minor merger is lost "wandering" in the remaining galaxy (motivated by the simulations of e.g. Bellovary et al 2019 and the observations of Reines et al 2020), and found that it makes very little difference on the overall AGN population.…”

Prospects to Explore High-redshift Black Hole Formation with Multi-band Gravitational Waves Observatories

“…Alternatively, heavy seeds with typical mass m • = 10 4 − 10 6 M could have formed by exotic pathways at z > 10 ( Bromm & Loeb 2003;Lodato & Natarajan 2006). While additional, alternative pathways to form massive BHs at high redshift have been described in the literature (see, e.g., Devecchi & Volonteri 2009;Davies et al 2011), including medium-weight seeds (Sassano et al 2021), the classic distinction between light and heavy seeds will be considered here for the sake of clarity. To date, it is still unknown whether light seeds, heavy seeds or, more likely, a combination of them contributed to the formation of the population of the first BHs.…”

Multi-flavour SMBH seeding and evolution in cosmological environments

“…Alternative models assume a more massive origin of SMBHs in order to relax the constraints on the seeds growthrates. Indeed, heavy or intermediate-mass BHs (IMBHs), forming with 10 3 M seed /M 10 5 already at z 10 allow to more easily reach M BH ∼ 10 8−9 M by z ∼ 7 (e.g., Bromm & Loeb 2003;Omukai et al 2008;Shang et al 2010;Agarwal et al 2012;Volonteri & Bellovary 2012;Regan et al 2014;Visbal et al 2014b;Valiante et al 2016;Sassano et al 2021). On the other hand, these models generally require peculiar physical conditions whose occurrence is yet poorly constrained over cosmological contexts (e.g., Bromm & Loeb 2003;Agarwal et al 2016;Latif et al 2018).…”

“…hν = [11.2 − 13.6] eV. stra et al 2014;Valiante et al 2016;Volonteri & Reines 2016;Lupi et al 2021b;Sassano et al 2021). The first ones can generally model directly the small-scale processes involved in the formation of BH seeds.…”

“…Nevertheless, due to the MR-II mass resolution, the physical scales at which PopIII stars form are inaccessible to L-Galaxies. Therefore, we exploit the results of the GQd model (Valiante et al 2011(Valiante et al , 2014(Valiante et al , 2016 where PopIII evolution and the formation of light seeds is followed self-consistently (see also Sassano et al 2021;Trinca et al 2022, for recent updates). In this way, we implement the formation of light seeds in a sub-grid fashion within the L-Galaxies code, while self-consistently tracking the formation of massive BH seeds.…”