Hyper-Eddington black hole growth in star-forming molecular clouds and galactic nuclei: can it happen?

Abstract

Formation of supermassive black holes (BHs) remains a theoretical challenge. In many models, especially beginning from stellar relic “seeds,” this requires sustained super-Eddington accretion. While studies have shown BHs can violate the Eddington limit on accretion disk scales given sufficient “fueling” from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding ISM. We explore this in a suite of multi-physics high-resolution simulations of BH growth in magnetized, star-forming dense gas complexes including dynamical stellar feedback from radiation, stellar mass-loss, and supernovae, exploring populations of seeds with masses ∼1 − 104 M⊙. In this initial study, we neglect feedback from the BHs: so this sets a strong upper limit to the accretion rates seeds can sustain. We show that stellar feedback plays a key role. Complexes with gravitational pressure/surface density below ∼103 M⊙ pc−2 are disrupted with low star formation efficiencies so provide poor environments for BH growth. But in denser cloud complexes, early stellar feedback does not rapidly destroy the clouds but does generate strong shocks and dense clumps, allowing $\sim 1\%$ of randomly-initialized seeds to encounter a dense clump with low relative velocity and produce runaway, hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass growth under these conditions is almost independent of initial BH mass, allowing rapid IMBH formation even for stellar-mass seeds. This defines a necessary (but perhaps not sufficient) set of criteria for runaway BH growth: we provide analytic estimates for the probability of runaway growth under different ISM conditions.

A Candidate for the Least-massive Black Hole in the First 1.1 Billion Years of the Universe

“…The gray dashed line shows our best-fit power-law continuum model, where the continuum slope index is α λ = −1.32 ± 0.30. massive BH can power this source if rapid super-Eddington accretion is achieved. The necessity of such intermittent super-Eddington phases has been recently argued in theoretical predictions of early BH assembly (Hu et al 2022;Inayoshi et al 2022a;Shi et al 2023). On the other hand, the same luminosity can also be achieved by a sub-Eddington BH with M BH ; 10 7−8 M e , as long as the nuclear accretion disk settles down to a radiatively efficient state (L bol /L Edd  0.01; see Yuan & Narayan 2014).…”

From Seeds to Supermassive Black Holes: Capture, Growth, Migration, and Pairing in Dense Protobulge Environments

“… , where t Sal = κ es c/(4πG) ≈ 45 Myr is the Salpeter time and ò ref is an assumed radiative efficiency (typically 0.1; Inayoshi et al 2020). Thus, to grow to SMBH masses from "seed" masses =10 6 M e (Greene et al 2020) at high redshifts generally requires either accretion well above this naive Eddington limit (Inayoshi et al 2016;Shi et al 2023) or exotic intermediatemass black hole (IMBH) seed formation scenarios like the direct collapse of unfragmented giant molecular clouds (GMCs) to single hypermassive stars (with SMBH masses themselves; Bromm & Loeb 2003), exotic dark matter/new physics processes (Xiao et al 2021), or runaway mergers of stars (Portegies Zwart et al 2004;Shi et al 2021;Rantala et al 2024).…”

“…Some of the problems above are explored in Shi et al (2023), where we embedded BH seeds into star-forming GMCs. We found that significant BH accretion can "stochastically" occur if a dense clump (parsec or subparsec scale) formed via turbulence and stellar feedback shocks (Klessen 2000; Mac Low & Klessen 2004;McKee & Ostriker 2007) is sufficiently gravitationally bound and close (subparsec) to the BH and if the gas is sufficiently dense and cold (Inayoshi et al 2016).…”

“…We found that significant BH accretion can "stochastically" occur if a dense clump (parsec or subparsec scale) formed via turbulence and stellar feedback shocks (Klessen 2000; Mac Low & Klessen 2004;McKee & Ostriker 2007) is sufficiently gravitationally bound and close (subparsec) to the BH and if the gas is sufficiently dense and cold (Inayoshi et al 2016). In practice, this "clump accretion" only occurs if both the gas and the BH dynamics are dominated by the GMC's self-gravity, and stellar feedback is globally inefficient (unable to rapidly unbind all the gas in the GMC as soon as the first massive stars form; Shi et al 2023), which is true for GMCs with high initial surface density (Grudić et al 2018b(Grudić et al , 2021aChevance et al 2023). In Shi et al (2024), we extended these to include physically motivated feedback from BH accretion in the forms of radiation (Jiang et al 2019), accretion disk winds and jets (Silk & Rees 1998;Blandford & Begelman 1999), and cosmic rays (Guo & Oh 2008;Guo & Mathews 2012;Zweibel 2017).…”

“…Here, we present a case study of one of the massive, highdensity cloud-complex simulations in Shi et al (2023Shi et al ( , 2024, representative of plausible starburst conditions in high-redshift progenitors of massive galaxies. We show that this demonstrates a number of remarkable phenomena that could unify many previously proposed mechanisms to enable rapid formation of a well-defined dynamical center, "trapping" of BHs, efficient and rapid migration of BH pairs to the center, efficient gas capture, and hyper-Eddington accretion through a hypermagnetized accretion disk.…”