Little Red Dots from Small-Scale Primordial Black Hole Clustering

2025/07/09 by B. Zhang, Zhang, Borui, Wei-Xiang Feng +3 · 3 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Gravitation Theories #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #General Relativity and Quantum Cosmology (gr-qc) #High Energy Physics - Phenomenology (hep-ph)

paper · pdf · doi:10.48550/arxiv.2507.07171

openalex publication_date 2025/07/09 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28

Abstract

The James Webb Space Telescope (JWST) observations have identified a class of compact galaxies at high redshifts (4 \lesssim z \lesssim 11), dubbed "little red dots" (LRDs). The supermassive black holes (SMBHs) of 10^5-8\rm M_\odot in LRDs favor a heavy-seed origin. We propose a mechanism for their formation: Clusters of primordial black holes, formed through long-short mode coupling on small scales in the early Universe, undergo sequential mergers over extended timescales. This mechanism can evade cosmic microwave background distortions and result in heavy-seed SMBHs via runaway mergers. We employ Monte Carlo simulations to solve the Smoluchowski coagulation equation and determine the runaway merging timescale. The resulting stochastic gravitational wave background offers a distinct signature of this process, and the forming SMBHs can be highly spinning at their formation due to the spin residual of the cluster from tidal fields. This mechanism may explain the rapidly spinning SMBHs in LRDs under the assumption of obscured active galactic nuclei.

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Cited by

• Primordial black holes: constraints, potential evidence and prospects
• Black Hole Cold Brew: Fermi Degeneracy Pressure
• Revealing the origin of supermassive black holes with Taiji-TianQin network

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