A Dark-matter Origin of Little Red Dots: Early Seeding and Super-Bondi Accretion

Hua-Peng Gu, Fangzhou Jiang, Xian Chen, Ran Li, Zi-Xiang Jia

First listed 2026-09-16 | Last updated 2026-09-14

Abstract

The "Little red dots" (LRDs) are a population of accreting supermassive black holes (SMBHs) in the early Universe which often exhibit undermassive or even undetectable stellar hosts. Their early emergence, high space density, and extremely large black-hole-to-stellar mass ratios pose a serious challenge to conventional seeding scenarios that rely on baryon for both the formation and growth of black holes. Here we demonstrate that the above anomalies can be naturally resolved if dark matter is self-interacting. We apply a fully relativistic, non-equilibrium halo-evolution model, first developed in our earlier work, to trace the complete gravothermal evolution of self-interacting dark matter (SIDM) halos, from the initial collapse into BH seeds to the subsequent accretion of dark matter. We find that in highly concentrated halos assembled before reionization, gravothermal collapse efficiently produces stellar-mass black-hole seeds within a few hundred million years. Remarkably, and contrary to standard expectations for dark-matter accretion, heat conduction in SIDM then sustains a prolonged super-Bondi inflow that drives these seeds to supermassive scale by the LRD epoch, without baryonic assistance. The halo conditions required for completing these processes, together with the probability of avoiding major mergers that disrupt gravothermal evolution, result in an SMBH population consistent with the observed abundance and redshift distribution of LRDs. Our findings establish a pathway in which SMBHs are seeded and assembled primarily from dark matter, well before substantial galaxies form around them, thereby offering both a compelling physical explanation for LRDs and a new observational probe of dark-matter microphysics.

Short digest

Gu et al. model little red dots as the descendants of self-interacting dark-matter halos whose gravothermal collapse forms stellar-mass black-hole seeds before reionization. In their fully relativistic, non-equilibrium calculations, the post-collapse SIDM core feeds the seed through near-free-fall and heat-conduction-supported super-Bondi inflow, allowing growth to supermassive scales without baryonic accretion. Combining the required halo concentrations with halo abundances and merger survival probabilities, they find an SMBH population consistent with the observed LRD abundance and redshift distribution. The work offers a dark-sector explanation for LRDs with weak stellar hosts, while making their demographics a potential probe of SIDM microphysics and of merger disruption in early halos.

Key figures to inspect

• Figure 1. Use this overview schematic to establish the paper's central causal chain: SIDM thermalization, gravothermal collapse, horizon formation, and conductive super-Bondi growth into an SMBH. It makes clear that the authors treat seed formation and subsequent dark accretion as one continuous halo-evolution problem.
• Figure 3. This is the key physical diagnostic for the paper's distinctive claim. The comparison with the Bondi prediction isolates the near-free-fall and heat-conduction-driven phases that keep dark-matter accretion super-Bondi for most of the relevant growth period.
• Figure 4. This figure connects the modeled black-hole growth tracks directly to spectroscopically measured LRD black-hole masses and shows how varying initial halo mass and concentration spans the observed distribution. It also visualizes the merger-interruption boundary and the possible extra growth enabled by smooth cosmological halo accretion.
• Figure 5. Use this parameter-space synthesis to show which pre-reionization halos can seed and grow SMBHs on time, and why the viable population occupies a preferred halo-mass range. The lower panel links the timing requirement to both merger survival and the halo mass function, which underpins the demographic argument.
• Figure 7. This figure provides the merger-history caveat behind the population calculation: low-mass halos are frequently disrupted by major mergers before completing gravothermal evolution. It explains why the mechanism does not simply operate in every early SIDM halo.