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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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This content will become publicly available on April 15, 2026
The Convergence of Heavy and Light Seeds to Overmassive Black Holes at Cosmic Dawn
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 (MBH/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 theMBH/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 theMBH–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.
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- Award ID(s):
- 2006176
- PAR ID:
- 10640008
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 983
- Issue:
- 2
- ISSN:
- 2041-8205
- Page Range / eLocation ID:
- L37
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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