Small-Scale Clustering of Primordial Black Holes: The Little Red Dot Mass Function and the High-Redshift Galaxy Tension

Borui Zhang, Wei-Xiang Feng, Haipeng An

First listed 2026-09-09 | Last updated 2026-09-16

Abstract

Supermassive black holes (SMBHs) in "little red dots" (LRDs) discovered by the James Webb Space Telescope (JWST) may result from runaway mergers of primordial black holes (PBHs) in clusters---through long-short mode coupling on small scales in the early Universe. In this framework, we derive the SMBH mass function, together with the compactness and overmassive features of LRDs. We also estimate that the dense gas residing in PBH clusters is consistent with LRD observations. In addition, SMBHs formed from PBH clusters can help accelerate galaxy formation at high redshifts, thus alleviating tension with $Λ$CDM cosmology.

Short digest

Zhang, Feng, and An develop a primordial-black-hole cluster scenario in which long–short mode coupling produces dense PBH clusters whose runaway mergers seed the supermassive black holes associated with little red dots. Including subsequent accretion, the model derives an SMBH mass function consistent with LRD observations and connects black-hole mass to host-halo, stellar-mass, and gas-disk properties, naturally producing overmassive and compact systems when LRDs occupy low-angular-momentum halo components. The same high-mass PBH-cluster tail supplies early seeds that can boost galaxy formation and reduce the reported high-redshift stellar-mass-density tension with ΛCDM.

Key figures to inspect

• Figure 1. This is the central population-level prediction: the SMBH mass function generated by PBH clusters through long–short mode coupling. It is the most direct figure for assessing the paper’s claim that, after accretion, the model can reproduce the LRD black-hole population.
• Figure 2. This figure shows the stellar-to-BH ratios in PBH-cluster-induced haloes across star-formation-rate assumptions. It directly illustrates the mechanism invoked for the unusually overmassive black holes inferred in little red dots.
• Figure 4. The effective-radius versus UV-luminosity relation provides the paper’s compactness diagnostic for seed-effect-induced galaxies. It is especially useful for connecting the PBH-cluster framework to the observed small sizes of LRD hosts.
• Figure 5. This cumulative stellar-mass-density comparison at z≈10 captures the paper’s broader cosmological payoff: early SMBH seeds from PBH clusters can accelerate halo and galaxy growth. The cutoff-redshift dependence also makes clear which modeling choice controls the proposed relief of the high-redshift galaxy tension.