More Like this

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. 

   more » « less
2. Hyper-Eddington black hole growth in star-forming molecular clouds and galactic nuclei: can it happen?

   https://doi.org/10.1093/mnras/stac3245  

   Shi, Yanlong; Kremer, Kyle; Grudić, Michael Y.; Gerling-Dunsmore, Hannalore J.; Hopkins, Philip F. (November 2022, Monthly Notices of the Royal Astronomical Society)

   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 disc scales given sufficient ‘fuelling’ from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding interstellar medium (ISM). We explore this in a suite of multiphysics 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 $$\sim 1\!-\!10^{4}\, \mathrm{M}_{\odot }$$. 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 $$\sim 10^{3}\, \mathrm{M}_{\odot }\, {\rm pc^{-2}}$$ 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 $$\sim 1{{\ \rm per\ cent}}$$ 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 intermediate-mass black hole (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. 

   more » « less
3. The Convergence of Heavy and Light Seeds to Overmassive Black Holes at Cosmic Dawn

   https://doi.org/10.3847/2041-8213/adc680  

   Hu, Haojie; Inayoshi, Kohei; Haiman, Zoltán; Ho, Luis C; Ohsuga, Ken (April 2025, The Astrophysical Journal Letters)

   Abstract The James Webb Space Telescope has revealed low-luminosity active galactic nuclei at redshifts ofz≳ 4–7, many of which host accreting massive black holes (BHs) with BH-to-galaxy mass (M_BH/M_⋆) ratios exceeding the local values by more than an order of magnitude. The origin of these overmassive BHs remains unclear but requires potential contributions from heavy seeds and/or episodes of super-Eddington accretion. We present a growth model coupled with dark matter halo assembly to explore the evolution of theM_BH/M_⋆ratio under different seeding and feedback scenarios. Given the gas inflow rates in protogalaxies, BHs grow episodically at moderate super-Eddington rates, and the mass ratio increases early on, despite significant mass loss through feedback. Regardless of seeding mechanisms, the mass ratio converges to a universal value ∼0.1–0.3, set by the balance between gas feeding and star formation efficiency in the nucleus. This behavior defines an attractor in theM_BH–M_⋆diagram, where overmassive BHs grow more slowly than their hosts, while undermassive seeds experience rapid growth before aligning with the attractor. We derive an analytical expression for the universal mass ratio, linking it to feedback strength and halo growth. The convergence of evolutionary tracks erases seeding information from the mass ratio byz∼ 4–6. Detecting BHs with ∼10⁵⁻⁶M_⊙at higher redshifts that deviate from the convergence trend would provide key diagnostics of their birth conditions. 

   more » « less
4. Growth of massive black holes in FFB galaxies at cosmic dawn

   https://doi.org/10.1051/0004-6361/202452393  

   Dekel, Avishai; Stone, Nicholas C; Chowdhury, Dhruba Dutta; Gilbaum, Shmuel; Li, Zhaozhou; Mandelker, Nir; van_den_Bosch, Frank C (March 2025, Astronomy & Astrophysics)

   Aims.The scenario of feedback-free starbursts (FFB), which predicts excessively bright galaxies at cosmic dawn as observed using JWST, may provide a natural setting for black hole (BH) growth. This involves the formation of intermediate-mass seed BHs and their runaway mergers into super-massive BHs with high BH-to-stellar mass ratios and low Active Galactic Nucleus (AGN) luminosities. Methods.We present a scenario of merger-driven BH growth in FFB galaxies and study its feasibility. Results.Black hole seeds form within the building blocks of the FFB galaxies, namely, thousands of compact star clusters, each starbursting in a free-fall time of a few million years before the onset of stellar and supernova feedback. The BH seeds form by rapid core collapse in the FFB clusters, in a few free-fall times, which is sped up by the migration of massive stars due to the young, broad stellar mass function and stimulated by a “gravo-gyro” instability due to internal cluster rotation and flattening. BHs of ∼10⁴ M_⊙are expected in ∼10⁶ M_⊙FFB clusters within sub-kiloparsec galactic disks atz​ ∼ ​10. The BHs then migrate to the galaxy center by dynamical friction, hastened by the compact FFB stellar galactic disk configuration. Efficient mergers of the BH seeds will produce ∼10^6 − 8 M_⊙BHs with a BH-to-stellar mass ratio ∼0.01 byz​ ∼ ​4 − 7, as observed. The growth of the central BH by mergers can overcome the bottleneck introduced by gravitational wave recoils if the BHs inspiral within a relatively cold disk or if the escape velocity from the galaxy is boosted by a wet compaction event. Such events, common in massive galaxies at high redshifts, can also help by speeding up the inward BH migration and by providing central gas to assist with the final parsec problem. Conclusions.The cold disk version of the FFB scenario provides a feasible route for the formation of supermassive BHs. 

   more » « less
5. 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. 

   more » « less