Little Red Dots from ultra-strongly self-interacting dark matter

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 ≳ 10 5 M ⊙ at redshifts z ≳ 5. We develop a semi-analytic model that tracks the evolution of the dark matter halo population, the redshift of collapse z 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 ∼ 5 shows excellent agreement with LRD observational data and SIDM merger-tree simulations, particularly at the high-mass end ( m BH ≳ 10 7 M ⊙ ). 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.