Formation of Heavy Seed Black Holes and Little Red Dots-like Compact Clusters in Metal-enriched Star-forming Regions

Sunmyon Chon, Shingo Hirano, Ke-Jung Chen, Volker Springel

First listed 2026-09-02 | Last updated 2026-09-02

Abstract

Short digest

Using a cosmological radiation-hydrodynamic zoom-in simulation with self-consistent rapidly accreting supermassive-star evolution, Chon et al. connect heavy black-hole seed formation to compact cluster assembly in strongly irradiated, weakly metal-enriched star-forming regions. The first seeds arise once the mass-weighted internal Lyman-Werner field reaches J21~1000, typically from gas at [Z/H]~−3 to −2; every halo above 10^9 Msun in the simulated region hosts a BH exceeding 10^5 Msun. Five of eight clusters above 10^6 Msun produce intrinsically V-shaped, Little Red Dot-like continua, with 6000 K bloated supermassive stars often supplying the red optical component and normal-population Balmer breaks adding to it. The result frames heavy seeds, dense compact clusters, and even off-center LRD-like sources as linked products of clustered, enriched-but-still-low-metallicity environments rather than exclusively pristine direct-collapse sites.

Key figures to inspect

• Figure 1. Maps the simulated metal-enriched, FUV-irradiated star-forming environment and locates the eight massive SMS-hosting clusters, directly visualizing the spatial setting proposed to unite heavy-seed formation with LRD-like compact systems.
• Figure 3. Shows that SMS formation occupies the overlap of strong external Lyman-Werner irradiation and low, but nonzero, stellar metallicity, making the paper's departure from a strictly pristine direct-collapse picture concrete.
• Figure 6. Quantifies the halo occupation fraction of heavy seed BHs and underpins the headline result that all halos above 10^9 Msun in the zoom-in region contain BHs more massive than 10^5 Msun.
• Figure 10. Presents the full spectral decomposition of all eight massive clusters into normal-star and SMS light, establishing how frequently the simulated systems generate the red optical excess associated with LRD-like V-shaped continua.
• Figure 13. Places the simulated UV and optical slopes against the empirical V-shaped LRD selection region, providing the clearest population-level diagnostic for the claimed LRD-like spectral resemblance and the effect of dust attenuation.