Little Red Dots from Small-Scale Primordial Black Hole Clustering

TL;DR

This paper proposes that primordial black hole clusters in the early universe can merge over time to form heavy-seed supermassive black holes, explaining observations of high-redshift compact galaxies with rapidly spinning SMBHs.

Contribution

It introduces a new formation mechanism for heavy-seed SMBHs via primordial black hole cluster mergers, supported by Monte Carlo simulations and gravitational wave predictions.

Findings

Primordial black hole clusters can undergo runaway mergers within cosmic timescales.

The process produces a stochastic gravitational wave background detectable by future observations.

Formed SMBHs can retain high spins due to tidal field effects during mergers.

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.