The Clustering of Little Red Dots from Ultra-Strongly Self-Interacting Dark Matter

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

First listed 2025-12-19 | Last updated 2025-12-19

Abstract

We predict the effective clustering bias parameter, $b_{\rm{eff}}$, at $z\sim5$ for Little Red Dots (LRDs) seeded by Ultra-Strongly Self-Interacting Dark Matter (uSIDM). From our model, we find that $b_{\rm{eff}}\sim4.5$, thus we infer that LRDs seeded by uSIDM would populate halos of typical masses $\sim 8\times10^{10}~M_{\odot}$; this bias factor is consistent with LRDs being a distinct population from high redshift quasars. To the extent that we are aware, this is the first formation-based theoretical prediction of LRD clustering from a model consistent with the LRD mass function. We find that this bias and clustering is insensitive to a wide range of the underlying uSIDM microphysics parameters, including the uSIDM cross-section $σ/m$ and uSIDM fraction $f$. This is therefore a robust prediction from the uSIDM model, and will allow for direct probes of the uSIDM paradigm as the origin of LRDs in the next few years. Upcoming \texttt{JWST} observations will constrain the population of LRDs, including directly measuring their clustering.

Short digest

This Letter computes the large‑scale clustering of Little Red Dots at z~5 assuming their SMBH seeds arise from gravothermal collapse in ultra‑strongly self‑interacting dark matter. Weighting the uSIDM‑predicted LRD mass function with the Tinker halo bias yields an effective bias b_eff ≈ 4.5, implying characteristic host halos of ~8×10^10 Msun and a population distinct from luminous high‑z quasars. The prediction is robust—b_eff is effectively insensitive to uSIDM microphysics (σ/m and fraction f) and independent of the assumed duty cycle—providing a clean observational target. Upcoming JWST samples can directly measure LRD clustering to test the uSIDM origin scenario.

Key figures to inspect

• Figure 1: Inspect how the uSIDM LRD mass function (colored points) compares to SIDM simulations and Kokorev et al. data; use the slope/normalization—and the horizontal Eddington‑ratio shift bars—to see which BH‑mass ranges dominate the number‑density weighting that feeds into b_eff.
• Figure 2: Read b_eff from the gray band of power‑law mass‑function fits and map it to a characteristic halo mass via the red Tinker bias curve; compare with the orange (Schindler et al.) and blue (Pizzati et al.) bands to gauge whether LRDs prefer lower‑mass halos than quasars.
• Figure 3: Check that b_eff from the full uSIDM mass functions clusters tightly around ~4.5 despite varied microphysics, confirming the stated insensitivity and the implied host‑halo mass near 8×10^10 Msun.