Formation of the Little Red Dots from the Core-collapse of Self-interacting Dark Matter Halos

Fangzhou Jiang, Zixiang Jiang, Haonan Zheng, Luis C. Ho, Kohei Inayoshi, Xuejian Shen, Mark Vogelsberger, Wei-Xiang Feng

First listed 2025-03-31 | Last updated 2025-08-01

Abstract

We present a statistical study of black hole (BH) formation and growth seeded by gravothermal core collapse of self-interacting dark matter (SIDM) halos at high redshift, using a cosmological semi-analytical framework based on Monte Carlo merger trees. We demonstrate that gravothermal collapse naturally leads to BH formation in high-concentration halos at a characteristic mass scale set by the SIDM cross section, and occurs predominantly in the early Universe. This mechanism is particularly promising for explaining the abundance of the little red dots (LRDs) -- a population of early, apparently galaxy-less active galactic nuclei hosting supermassive BHs. By incorporating this seeding process with simple models of BH growth and assuming a 100% duty cycle, we reproduce the observed LRD mass function for velocity-dependent cross sections of $σ_{0m} \sim 30\,\mathrm{cm}^2\,\mathrm{g}^{-1}$ and $ω\sim 80\,\mathrm{km}\,\mathrm{s}^{-1}$, which are consistent with independent constraints from local galaxies. While higher values of $σ_{0m}$ (or $ω$) would overpredict the low-mass (or high-mass) end of the BH mass function, such deviations could be reconciled by invoking a reduced duty cycle or lower Eddington ratio. Our results suggest that the demographics of high-redshift BHs can serve as a novel and complementary probe of SIDM physics.

Short digest

Using Monte Carlo merger trees, the authors model black hole seeding via gravothermal core-collapse in self-interacting dark matter halos and their subsequent growth at early times. Collapse occurs in rare, high-concentration halos at a characteristic mass scale set by the velocity-dependent cross section, naturally producing seeds early enough to explain compact, galaxy-poor little red dots. With simple growth assumptions and a 100% duty cycle, the model reproduces the observed LRD black hole mass function for σ0m ~ 30 cm^2 g^-1 and ω ~ 80 km s^-1, consistent with independent local constraints. If σ0m or ω are higher, the resulting overproduction at one end of the mass function can be mitigated by a reduced duty cycle or lower Eddington ratios, suggesting high‑z BH demographics as a probe of SIDM physics.

Key figures to inspect

• Figure 1: Read the c–M maps to see where halos lie above the collapse-time contour by the LRD epoch; this shows the need for very high concentrations and reveals the cross‑section–dependent characteristic mass that collapses fastest (aligned with rare 2–3σ peaks).
• Figure 2: Track the blue collapse-time curves against the EPS look‑back distributions; the red highlighted early-forming tail pinpoints which halo masses and redshifts actually seed BHs, emphasizing that seeding only operates at high z and within a narrow mass window.
• Figure 3: Inspect the VVV‑L1 concentration distribution and its high‑c tail (84–99th percentiles) to judge whether the concentrations required in Fig. 1 are plausible and how often such progenitors appear in merger trees.
• Figure 4: Compare the modeled BH mass function (median/mean and scatter) with the LRD-inferred points from Kokorev et al.; check how the chosen σ0m and ω reproduce the normalization and shape, and where deviations hint at duty cycle or Eddington-ratio adjustments.