Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

M. Grant Roberts, Lila Braff, Aarna Garg, Stefano Profumo, Tesla Jeltema

First listed 2025-07-04 | Last updated 2026-03-06

Abstract

We investigate the possibility that the recently identified population of high-redshift, obscured quasars - known as "Little Red Dots" (LRDs) - originates from early black hole seed formation driven by ultra-strongly self-interacting dark matter (uSIDM). In this framework, dark matter halos undergo gravothermal core collapse due to large self-interaction cross sections, resulting in the rapid formation of massive black hole (BH) seeds with masses $\gtrsim 10^{5} M_\odot$ at redshifts $z \gtrsim 5$. We develop a semi-analytic model that tracks the evolution of the dark matter halo population, the redshift of collapse $z_{\rm coll}$, and the corresponding BH mass function. Black hole growth is modeled stochastically via a log-normal Eddington ratio distribution and a finite duty cycle. We find that the uSIDM scenario naturally reproduces key observed properties of LRDs, including their abundance, compactness, and characteristic BH masses, while offering a mechanism for early, obscured black hole formation that is difficult to achieve in standard CDM-based models. The predicted SMBH mass function at $z \sim 5$ shows excellent agreement with LRD observational data and SIDM merger-tree simulations, particularly at the high-mass end $(m_{\rm BH} \gtrsim 10^{7} M_\odot)$. These results suggest that LRDs may serve as powerful observational tracers of exotic dark sector physics and that SMBH formation in the early universe could be significantly shaped by non-gravitational dark matter interactions.

Short digest

Proposes ultra-strongly self-interacting dark matter (uSIDM) as a route to rapid gravothermal core collapse in early halos, yielding massive black hole seeds (≳10^5 Msun) by z≳5 that can power Little Red Dots. Builds a semi-analytic framework tracking halo evolution, collapse redshifts z_coll, and the SMBH mass function with stochastic growth via a log-normal Eddington ratio and finite duty cycle. Finds that uSIDM reproduces LRD abundance, compactness, and characteristic BH masses, with a z≈5 SMBH mass function matching LRD data and SIDM merger-tree results, especially at m_BH≳10^7 Msun. Implies LRDs could be incisive tracers of non-gravitational dark sector physics and that early SMBH assembly may depart from CDM expectations.

Key figures to inspect

• SMBH mass function at z≈5 overlaid with LRD-inferred number densities and a SIDM merger-tree comparison—inspect agreement at the high-mass end (m_BH≳10^7 Msun) and any residuals at lower masses.
• Redshift of gravothermal collapse (z_coll) versus halo mass and uSIDM cross-section—use this to see what parameter space yields seeds ≳10^5 Msun by z≳5 and the sensitivity to cross-section strength.
• LRD abundance/compactness predictions versus redshift—compare model number densities and effective sizes to JWST LRD counts to verify the claimed match in prevalence and compactness.
• Growth prescription diagnostics: distribution of Eddington ratios and duty cycle and their impact on luminosity functions—check how sub-Eddington, intermittent growth reproduces LRD luminosities while remaining X-ray faint.
• Parameter variation panel: how changing the uSIDM cross-section or duty cycle shifts the predicted BH mass function—identify degeneracies and ranges still consistent with LRD observables.