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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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This content will become publicly available on March 1, 2026
Growth of massive black holes in FFB galaxies at cosmic dawn
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 ∼106 − 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.
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- Award ID(s):
- 2407063
- PAR ID:
- 10596385
- Publisher / Repository:
- EDP Sciences
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 695
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A97
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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