Evolution of High-redshift Quasar Hosts and Promotion of Massive Black Hole Seed Formation

Abstract

High-redshift luminous quasars powered by accreting supermassive black holes (SMBHs) with mass ≳109 M ⊙ constrain their formation pathways. We investigate the formation of heavy seeds of SMBHs through gas collapse in the quasar host progenitors, using merger trees to trace the halo growth in highly biased, overdense regions of the universe. The progenitor halos are likely irradiated by intense H2-photodissociating radiation from nearby star-forming galaxies and heat the interior gas by successive mergers. The kinetic energy of the gas originating from mergers, as well as the baryonic streaming motion, prevents gas collapse and delays prior star formation. With a streaming velocity higher than the rms value, gas clouds in nearly all 104 realizations of merger trees enter the atomic-cooling stage and begin to collapse isothermally with T ≃ 8000 K via Lyα cooling. The fraction of trees that host isothermal gas collapse is 14% and increases with streaming velocity, while the rest form H2-cooled cores after short isothermal phases. If the collapsing gas is enriched to Z crit ∼ 2 × 10−3 Z ⊙, requiring efficient metal mixing, this fraction could be reduced by additional cooling via metal fine-structure lines. In the massive collapsing gas, the accretion rate onto a newly born protostar ranges between 3 × 10−3 M ⊙ yr−1 and 5 M ⊙ yr−1, among which a large fraction exceeds the critical rate suppressing stellar radiative feedback. As a result, we expect a distribution of stellar mass (presumably BH mass) ranging from several hundred to above 105 M ⊙, potentially forming massive BH binary mergers and yielding gravitational-wave events.

Supercritical growth pathway to overmassive black holes at cosmic dawn: coevolution with massive quasar hosts

“…The BH growth model presented here focuses on highly biased regions of the universe where high-z quasar hosts harbored in massive DM halos with M h ∼ 10 12 M form by z ∼ 6. Such massive halos are expected to be ideal sites for the formation of massive seed BHs owing to their peculiar environments, e.g., strong ultra-violet irradiation from nearby galaxies and violent galaxy mergers (also see Li et al 2021;Lupi et al 2021) and sufficiently feed the central regions via intense cold gas streams (Di Matteo et al 2012). We find that the galactic gas inflows triggered during their assembly promote rapid growth of seed BHs in the protogalactic nuclei, nearly independent of their initial mass.…”

“…Here, we estimate this value as the baryonic mass growth rate of a massive dark matter (DM) halo that ends up as a high-z quasar host galaxy with mass of M h ∼ 10 12 M at z 6. Following Li et al (2021), we construct merger trees to track the growth of the DM halos in highly-biased, overdense regions of the universe and plant a seed BH with M BH,0 at z = z 0 in each tree.…”

“…In Fig. 1, we present the baryonic mass inflow rate into the progenitor DM halos defined by Ṁh (Ω b /Ω m ) for 10 4 different trees obtained in Li et al (2021), where Ω b /Ω m = 0.156 is the baryon fraction (Planck Collaboration 2018). We highlight three representative trees (red, orange, and blue) and overlay the median value In the fiducial case (p = 0.5; solid), the BH mass in all the cases (three representative trees and median tree) converge to MBH ∼ 10 9 M by z = 6, while the evolutionary tracks show a great diversity in the earlier stage depending on the halo merger assembly process.…”

“…The former case corresponds to a remnant BH originating from a firstgeneration star (Population III star, hereafter PopIII star), while the latter case mimics a heavy BH seed through supermassive star formation in massive DM halos under peculiar environments (Dijkstra et al 2008;Inayoshi et al 2018a;Wise et al 2019;Lupi et al 2021). A semi-analytical study by Li et al (2021) suggests that the formation of BH seeds is rather promoted in the high-z quasar progenitor halos located in the overdense regions and yields a BH mass distribution ranging from several hundred to above 10 5 M . We study the two scenarios that bracket the low and high mass ends for BH seeds.…”

Long-term evolution of supercritical black hole accretion with outflows: a subgrid feedback model for cosmological simulations

“…It is possible for these seeds to grow into SMBHs within ∼ 1 Gyr if the BHs continuously accrete at or slightly above (Tanaka & Haiman 2009;Madau et al 2014) the Eddington rate of ṀEdd [≡ L Edd /(0.1c 2 )], where L Edd is the Eddington luminosity and c is the speed of light. An alternative approach is to begin with heavier BH seeds with ∼ 10 4 − 10 6 M via collapse of supermassive stars (SMSs) or runaway stellar mergers in a dense star cluster (Bromm & Leob 2003;Omukai et al 2008;Devecchi & Volonteri 2009;Inayoshi et al 2014;Hirano et al 2017;Chon & Omukai 2020;Li et al 2021). Under special circumstances with strong H 2 photo-dissociating radi-ation produced from nearby galaxies, violent successive halo mergers, or high baryon-dark matter streaming motion, massive seed BHs are expected to form efficiently and get a head start toward the SMBH regime (Omukai 2001;Dijkstra et al 2008;Tanaka & Li 2014;Inayoshi et al 2018a;Wise et al 2019).…”