A cosmologist's take on Little Red Dots

Valerio De Luca, Loris Del Grosso, Gabriele Franciolini, Konstantinos Kritos, Emanuele Berti, Daniel D'Orazio, Joseph Silk

First listed 2025-12-22 | Last updated 2025-12-22

Abstract

The James Webb Space Telescope (JWST) has uncovered a population of compact, high-redshift sources, the Little Red Dots (LRDs), which may host supermassive black holes (BHs) significantly heavier than their stellar content compared with local scaling relations. These objects challenge standard models of early galaxy formation and may represent an extreme class of early BH hosts. In this paper, we investigate whether these BHs could have a primordial origin. We first show that the direct formation of these BH masses in the early Universe is excluded by stringent CMB $μ$-distortion limits. We then investigate the assembly of massive BHs from lighter, observationally allowed primordial black holes (PBHs) via hierarchical mergers, finding that, although this channel can operate depending on the merger history, it faces challenges in explaining the observations due to the rarity of the required high-redshift dark matter halos. Finally, we estimate gas accretion onto intermediate-mass PBHs, while jointly tracking metallicity evolution, and identify regions of parameter space in which such growth could reproduce the observed properties of LRDs. As a special case, we focus on the strongly lensed source QSO1, whose extremely low metallicity and large mass provide a stringent test of these formation channels.

Short digest

Tests whether the overmassive black holes inferred in JWST Little Red Dots can be primordial, comparing three pathways: direct PBH formation, hierarchical PBH mergers, and gas-fed growth of intermediate‑mass PBH seeds with co-evolving metallicity. Direct formation at LRD masses is excluded by stringent CMB μ‑distortion limits, and even slightly lighter masses face PTA-induced scalar‑GW constraints. Hierarchical mergers can work only in rare, deep high‑z halos, struggling to reach the implied LRD number densities. An accretion–metallicity model identifies viable regions where intermediate‑mass PBHs grow to match LRD properties—including the lensed, ultra‑low‑metallicity QSO1—making PBH‑seeded gas accretion the most promising primordial route.

Key figures to inspect

• Figure 1: Use the upper panel to see how μ‑distortion bounds rule out direct PBH formation at LRD‑scale masses; in the lower panel compare the PBH abundance limits against the light‑blue LRD number‑density band to gauge feasibility.
• Figure 2: Track democratic (red) versus oligarchic (blue) merger channels versus redshift and PBH fraction; vary the NFW concentration band to see that only extreme concentrations/high f_PBH approach the LRD box, underscoring the rarity problem.
• Figure 3: Read off which combinations of the fragmentation and accretion coefficients reproduce QSO1’s M_BH, M_* and very low Z; compare contours for different ejection parameters and follow the inset growth tracks versus redshift relative to the dashed metallicity ceiling.
• Figure 4: Examine how hierarchical‑merger probability rises with escape velocity; the sigmoid fit (Eq. 5) quantifies the deep‑potential threshold needed to retain remnants and sustain chaining mergers.