The Road to Normalcy: Environment-Driven Evolutionary Pathways for Primordial Black Holes

Saiyang Zhang, Junehyoung Jeon, Boyuan Liu, Volker Bromm, Priyamvada Natarajan

First listed 2026-08-25 | Last updated 2026-08-24

Abstract

We investigate how cosmological environment regulates the evolution of primordial black hole (PBH) seeds in the early universe using a suite of hydrodynamical simulations. Unlike traditional seeding channels, PBHs provide early BH seeds without the need of special conditions for gas collapse/fragmentation that favor overdense regions, thus covering diverse large-scale environments. We find that PBHs follow distinct evolutionary pathways depending on the gas supply and halo assembly history. In underdense regions, limited inflow suppresses both accretion and star formation, producing faint, metal-poor systems, while in overdense environments, sustained gas inflow drives rapid BH growth and the formation of compact, centrally concentrated stellar components. These differences lead to large variations in BH-to-stellar mass ratio, metallicity, and morphology, collectively bracketing a range of possible evolutionary pathways for PBH-seeded systems. We show that compact, BH-dominated sources resembling recently observed Little Red Dots naturally arise as one phase within these evolutionary pathways before evolving into more extended galaxy--AGN systems. Our results suggest that both the initial seed properties and cosmological environment jointly shape early BH--galaxy co-evolution, while subsequent environmental regulation can erase the memory of the initial seeding channel.

Short digest

Using cosmological hydrodynamical simulations, Zhang et al. follow primordial-black-hole-seeded hosts across isolated and overdense environments to isolate how gas supply and halo assembly regulate early BH--galaxy co-evolution. Underdense systems receive too little inflow to sustain accretion or star formation, remaining faint, metal-poor, and strongly BH-dominated, whereas overdense hosts build stars rapidly around more burstily fueled BHs. The resulting tracks span large differences in BH-to-stellar mass ratio, metallicity, compactness, and morphology, with LRD-like compact, BH-dominated sources emerging as a transient stage rather than a unique PBH signature. Continued assembly in dense environments can then dilute the initial overmassive-BH signature and move these systems toward more familiar extended galaxy--AGN configurations.

Key figures to inspect

• Figure 1. This directly connects the environmental premise to the mechanism: overdense runs have stronger virial-scale gas and dark-matter inflow and correspondingly burstier PBH fueling than the isolated case.
• Figure 2. This is the central evolutionary synthesis, placing the simulated BH-to-stellar and BH-to-halo mass-ratio tracks against LRD-like parameter space, local relations, DCBH models, and individual high-redshift BH candidates.
• Figure 5. This figure pairs stellar assembly with gas metallicity, showing why dense PBH hosts can transition away from chemically young, BH-dominated configurations while isolated or feedback-regulated systems remain weakly evolved.
• Figure 6. The projected stellar half-mass radii test the compactness component of the LRD comparison and explicitly show how the intrinsic host sizes relate to an approximate JWST/NIRCam resolution scale.
• Figure 7. This time sequence makes the proposed road to normalcy visually concrete: rapid stellar assembly and mergers transform an initially compact, irregular PBH-centered system into an extended galaxy even though the BH mass changes comparatively little.