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

   https://doi.org/10.3847/2041-8213/ad5a95  

   Shi, Yanlong; Kremer, Kyle; Hopkins, Philip F (July 2024, The Astrophysical Journal Letters)

   Abstract The origins and mergers of supermassive black holes (SMBHs) remain a mystery. We describe a scenario from a novel multiphysics simulation featuring rapid (≲1 Myr) hyper-Eddington gas capture by a ∼1000M_⊙“seed” black hole (BH) up to supermassive (≳10⁶M_⊙) masses in a massive, dense molecular cloud complex typical of high-redshift starbursts. Due to the high cloud density, stellar feedback is inefficient, and most of the gas turns into stars in star clusters that rapidly merge hierarchically, creating deep potential wells. Relatively low-mass BH seeds at random positions can be “captured” by merging subclusters and migrate to the center in ∼1 freefall time (vastly faster than dynamical friction). This also efficiently produces a paired BH binary with ∼0.1 pc separation. The centrally concentrated stellar density profile (akin to a “protobulge”) allows the cluster as a whole to capture and retain gas and build up a large (parsec-scale) circumbinary accretion disk with gas coherently funneled to the central BH (even when the BH radius of influence is small). The disk is “hypermagnetized” and “flux-frozen”: dominated by a toroidal magnetic field with plasmaβ∼ 10⁻³, with the fields amplified by flux-freezing. This drives hyper-Eddington inflow rates ≳1M_⊙yr⁻¹, which also drive the two BHs to nearly equal masses. The late-stage system appears remarkably similar to recently observed high-redshift “little red dots.” This scenario can provide an explanation for rapid SMBH formation, growth, and mergers in high-redshift galaxies. 

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2. Beyond Hierarchical Mergers: Accretion-driven Origins of Massive, Highly Spinning Black Holes in Dense Star Clusters

   https://doi.org/10.3847/2041-8213/ae1eeb  

   Kıroğlu, Fulya; Kremer, Kyle; Rasio, Frederic A (November 2025, The Astrophysical Journal Letters)

   Abstract GW231123, the most massive binary black hole (BBH) merger detected by LIGO–Virgo–KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called “hierarchical merger” channel), most star clusters with masses ≲10⁶M_⊙have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH–star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. UsingN-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process, and we find that accretion can drive BH masses up to at least ∼200M_⊙, with spins set by the details of the growth history. BHs up to about 150M_⊙can reach dimensionless spinsχ ≳ 0.7 via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down toχ ≲ 0.4. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to ∼10% of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems. 

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3. Heavy Seeds and the First Black Holes: Insights from the BRAHMA Simulations

   https://doi.org/10.3847/1538-4357/ae2607  

   Bhowmick, Aklant K; Blecha, Laura; Torrey, Paul; Kelley, Luke Zoltan; Natarajan, Priyamvada; Somerville, Rachel S; Weinberger, Rainer; Garcia, Alex M; Hernquist, Lars; Di_Matteo, Tiziana; et al (January 2026, The Astrophysical Journal)

   Abstract From the luminous quasars atz∼ 6 to the recentz∼ 9–11 active galactic nuclei (AGN) revealed by JWST, observations of the earliest black hole (BH) populations can provide unique constraints on BH evolution. We use theBRAHMAsimulations with constrained initial conditions to investigate BH assembly in extreme overdense regions. The simulations implement heavy ∼10⁴–10⁵M_⊙seeds forming in dense, metal-poor gas exposed to sufficient Lyman–Werner flux. With gas accretion modeled via the Bondi–Hoyle formalism and BH dynamics with a subgrid dynamical friction scheme, we isolate the impact of seeding, dynamics, accretion, and feedback on BH evolution. With fiducial stellar and AGN feedback inherited fromIllustrisTNG, accretion is suppressed atz≳ 9, leaving mergers as the dominant growth channel. Gas accretion dominates atz≲ 9, where permissive models (super-Eddington or low radiative efficiency) build ∼10⁹M_⊙BHs powering quasars byz∼ 6, while stricterIllustrisTNG-based prescriptions yield much smaller BHs (∼10⁶–10⁸M_⊙). Our seed models strongly affect mergers atz≳ 9: only the most lenient models (with ∼10⁵M_⊙seeds) produce enough BH mergers to reach ≳10⁶M_⊙byz∼ 10, consistent with current estimates for GN-z11. Our dynamical friction model gives low merger efficiencies. Therefore, even in such extreme regions, we are unable to produce ≳10⁷M_⊙BHs byz∼ 9–10, as currently inferred for GHZ9, UHZ1, and CAPERS-LRD-z9. If the BH-to-stellar mass ratios of these sources are indeed so extreme, they would require either very short BH merger timescales or reduced AGN thermal feedback. Weaker stellar feedback boosts both star formation and BH accretion and cannot raise these ratios. 

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4. Feedback-regulated seed black hole growth in star-forming molecular clouds and galactic nuclei

   https://doi.org/10.1051/0004-6361/202450964  

   Shi, Yanlong; Kremer, Kyle; Hopkins, Philip F (November 2024, Astronomy & Astrophysics)

   Context.The detection of supermassive black holes (SMBHs) in high-redshift luminous quasars may require a phase of rapid accretion, and as a precondition, substantial gas influx toward seed black holes (BHs) from kiloparsec or parsec scales. Our previous research demonstrated the plausibility of such gas supply for BH seeds within star-forming giant molecular clouds (GMCs) with high surface density (∼10⁴ M_⊙ pc⁻²), facilitating “hyper-Eddington” accretion via efficient feeding by dense clumps, which are driven by turbulence and stellar feedback. Aims.This article presents an investigation of the impacts of feedback from accreting BHs on this process, including radiation, mechanical jets, and highly relativistic cosmic rays. Methods.We ran a suite of numerical simulations to explore diverse parameter spaces of BH feedback, including the subgrid accretion model, feedback energy efficiency, mass loading factor, and initial metallicity. Results.Using radiative feedback models inferred from the slim disk, we find that hyper-Eddington accretion is still achievable, yielding BH bolometric luminosities of as high as 10⁴¹ − 10⁴⁴ erg/s, depending on the GMC properties and specific feedback model assumed. We find that the maximum possible mass growth of seed BHs (ΔM^max_BH) is regulated by the momentum-deposition rate from BH feedback,ṗ_feedback/(Ṁ_BHc), which leads to an analytic scaling that agrees well with simulations. This scenario predicts the rapid formation of ∼10⁴M_⊙intermediate-massive BHs (IMBHs) from stellar-mass BHs within ∼1 Myr. Furthermore, we examine the impacts of subgrid accretion models and how BH feedback may influence star formation within these cloud complexes. 

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5. The Assembly of the First Massive Black Holes

   https://doi.org/10.1146/annurev-astro-120419-014455  

   Inayoshi, Kohei; Visbal, Eli; Haiman, Zoltán (August 2020, Annual Review of Astronomy and Astrophysics)

   null (Ed.)

   The existence of ∼10 9 M ⊙ supermassive black holes (SMBHs) within the first billion years of the Universe has stimulated numerous ideas for the prompt formation and rapid growth of black holes (BHs) in the early Universe. Here, we review ways in which the seeds of massive BHs may have first assembled, how they may have subsequently grown as massive as ∼10 9 M ⊙ , and how multimessenger observations could distinguish between different SMBH assembly scenarios. We conclude the following: ▪  The ultrarare ∼10 9 M ⊙ SMBHs represent only the tip of the iceberg. Early BHs likely fill a continuum from the stellar-mass (∼10M ⊙ ) to the supermassive (∼10 9 ) regimes, reflecting a range of initial masses and growth histories. ▪  Stellar-mass BHs were likely left behind by the first generation of stars at redshifts as high as ∼30, but their initial growth typically was stunted due to the shallow potential wells of their host galaxies. ▪  Conditions in some larger, metal-poor galaxies soon became conducive to the rapid formation and growth of massive seed holes, via gas accretion and by mergers in dense stellar clusters. ▪  BH masses depend on the environment (such as the number and properties of nearby radiation sources and the local baryonic streaming velocity) and on the metal enrichment and assembly history of the host galaxy. ▪  Distinguishing between assembly mechanisms will be difficult, but a combination of observations by the Laser Interferometer Space Antenna (probing massive BH growth via mergers) and by deep multiwavelength electromagnetic observations (probing growth via gas accretion) is particularly promising. 

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