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

This study uses hydrodynamical simulations to demonstrate that the cosmological environment, rather than just initial seed properties, dictates the evolutionary pathways of primordial black holes, producing diverse galaxy-AGN systems that can naturally explain observations like Little Red Dots while eventually erasing the memory of their specific seeding mechanism.

Original authors: Saiyang Zhang, Junehyoung Jeon, Boyuan Liu, Volker Bromm, Priyamvada Natarajan

Published 2026-08-25

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CC BY 4.0

Original authors: Saiyang Zhang, Junehyoung Jeon, Boyuan Liu, Volker Bromm, Priyamvada Natarajan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the earliest moments of the universe, long before the first stars ignited, a mysterious population of objects may have formed: primordial black holes. Unlike the black holes we know today, which are born from the violent deaths of massive stars, these ancient seeds could have emerged directly from the dense, chaotic soup of the Big Bang itself. For decades, astronomers have wondered how these seeds grew into the supermassive giants that now sit at the centers of most galaxies. A major puzzle has been why some of the earliest galaxies observed by the James Webb Space Telescope contain black holes that seem far too heavy for their host galaxies, defying the rules that govern the relationship between black holes and stars in the nearby universe. Understanding how these ancient seeds evolved requires looking not just at the black holes themselves, but at the specific cosmic neighborhoods they inhabited, as the environment around them dictates whether they remain lonely, faint objects or grow into the engines of bright, active galaxies.

A team of researchers has now used powerful computer simulations to trace the life stories of these primordial black holes from the dawn of time to the era when the first galaxies were taking shape. By placing these ancient seeds into different cosmic environments within their simulations, the scientists discovered that the fate of a black hole is not written in stone at its birth. Instead, the surrounding gas and the history of the galaxy forming around it act as a powerful regulator, steering the black hole down one of several distinct evolutionary paths. The study suggests that the strange, extreme systems we see in the early universe are not necessarily a different species of object, but rather a temporary phase in a journey that can lead to the familiar galaxy-black hole partnerships we see today.

The researchers set up their digital universe to test how a primordial black hole behaves when placed in two very different settings. In one scenario, the black hole sits in a relatively empty, average region of space, far from other massive structures. In the other, it is placed in a dense, crowded region where matter is collapsing rapidly to form a massive galaxy. The simulations showed that in the empty regions, the black hole struggles to find fuel. With very little gas flowing toward it, the black hole grows slowly, and the surrounding gas fails to form many stars. The result is a faint, metal-poor system where the black hole remains overwhelmingly massive compared to the tiny, dim galaxy of stars that manages to form around it. In these lonely environments, the black hole essentially starves, and the system remains a quiet, underdeveloped relic.

In stark contrast, the simulations of the dense environments told a different story. Here, the black hole is surrounded by a rich supply of gas and is part of a rapidly assembling galaxy. The constant flow of matter allows the black hole to feed, but more importantly, it allows the surrounding stars to form in great numbers. The researchers found that in these crowded regions, the galaxy grows much faster than the black hole. Over time, the mass of the stars catches up to and eventually surpasses the mass of the black hole, shifting the balance from a black-hole-dominated system to a more standard galaxy. This process effectively "erases" the extreme signature of the initial seed, transforming a strange, overmassive object into a conventional galaxy with a central black hole.

One of the most striking findings is how this transformation explains the appearance of "Little Red Dots," a class of compact, red sources recently discovered by the James Webb Space Telescope. The simulations suggest that these objects are likely a specific phase in the life of a black hole in a dense environment. At a certain point, the galaxy is still small and compact, and the black hole is so bright and massive relative to the stars that it dominates the view. As the galaxy continues to grow and spread out, the black hole becomes a smaller part of the whole picture, and the system evolves into the more extended galaxies we see at later times. The study indicates that the Little Red Dots are not a permanent state, but a fleeting moment in the "road to normalcy" for these early systems.

The researchers also explored what happens when the starting seed is smaller. They found that lighter seeds in dense environments tend to remain weak and are quickly overwhelmed by the growth of the surrounding stars. Depending on exactly where the seed was placed within the forming galaxy, it might end up as a faint, off-center object or a quiet, embedded source that is difficult to distinguish from the rest of the galaxy. This variety in outcomes highlights that there is no single path for these ancient black holes. Some remain exotic and isolated, while others rapidly evolve into the familiar structures of the universe. The key factor is the environment: the supply of gas and the history of the galaxy's assembly determine whether the memory of the primordial seed is preserved or lost.

Ultimately, this work suggests that the extreme black holes seen in the early universe are not necessarily evidence of a completely different formation process, but rather a natural consequence of how these seeds interact with their surroundings. The simulations show that the initial conditions of the seed matter less than the subsequent growth of the galaxy. If a black hole is born in a rich, dense environment, the rapid assembly of stars can quickly bring the system into balance, making it look like a standard galaxy-AGN pair. However, if the seed is in a poor, isolated region, it remains an extreme outlier. The study concludes that to truly understand these ancient objects, astronomers must look at the entire system—the stars, the gas, and the environment—rather than just the black hole alone, as the environment holds the key to unlocking their evolutionary history.

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

Problem Statement
Recent James Webb Space Telescope (JWST) observations have revealed a diverse population of early galaxies and black holes (BHs) at high redshifts ( $z \gtrsim 10$ ), including massive galaxies, luminous Active Galactic Nuclei (AGN) candidates, and compact "Little Red Dots" (LRDs) with potentially overmassive BHs. A central question remains: how do early BHs and their baryonic environments co-evolve from diverse initial conditions toward the familiar galaxy-AGN scaling relations observed in the local universe? While Primordial Black Holes (PBHs) offer a theoretical pathway for early BH seeding independent of stellar collapse, it is unclear whether the signatures of PBH seeds (e.g., extreme BH-to-stellar mass ratios) persist over cosmic time or are erased by subsequent environmental regulation. This work investigates how the large-scale cosmological environment regulates the evolutionary pathways of PBH-seeded systems, determining whether they remain exotic, transiently appear as compact BH-dominated sources, or evolve into conventional galaxy-AGN systems.

Methodology
The authors employ cosmological hydrodynamical simulations using the GIZMO code, which combines a TreePM gravity solver with Lagrangian meshless finite-mass (MFM) hydrodynamics. The simulations utilize Planck18 cosmological parameters and a 12-species non-equilibrium primordial chemistry network, including fine-structure cooling and subgrid models for star formation, metal enrichment, and feedback.

Key methodological components include:

Initial Conditions: Simulations are run in a comoving box of $L = 2 \text{ cMpc}/h$ . The authors vary the large-scale environment by placing PBH seeds in two distinct regimes: (1) an approximately average-density region (isolated case) and (2) an overdense Lagrangian patch destined to collapse into a massive halo.
Seed Masses: The study focuses on fiducial seed masses of $10^7 M_\odot$ and $10^5 M_\odot$ , motivated by scenarios where massive PBHs arise from early-Universe phase transitions.
Feedback Physics: BH growth is tracked via Bondi–Hoyle–Lyttleton accretion with thermal feedback (coupling efficiency $\epsilon_r = 0.005$ ). Star formation follows a metallicity-dependent threshold ( $Z_{\text{th}} = 10^{-4} Z_\odot$ ) distinguishing Population III and II stars, with associated supernova feedback and metal enrichment.
Simulation Sets: The study includes full-volume runs and refined-region ("zoom-in") realizations to isolate environmental effects while maintaining high resolution on the PBH-hosting region. Runs span from $z \sim 3400$ (dark-matter-only) to $z \sim 3-12$ (hydrodynamical).

Key Contributions and Results

Environment-Dependent Evolutionary Pathways:
The simulations demonstrate that the cosmological environment is the primary regulator of PBH evolution.
- Isolated/Average-Density Regions: Limited gas inflow and strong accretion feedback suppress sustained star formation. These systems evolve into faint, metal-poor, and weakly fueled objects with extremely high BH-to-stellar mass ratios ( $M_\bullet/M_\star \gtrsim 10^4$ ), remaining significantly more BH-dominated than observed high-redshift systems.
- Overdense Regions: Sustained gas inflow and hierarchical assembly drive rapid stellar growth and BH accretion. In these environments, the BH-to-stellar mass ratio decreases by several orders of magnitude as the host galaxy assembles. Systems born in highly overdense regions with $10^7 M_\odot$ seeds can reach $M_\bullet/M_\star \sim 0.1$ by $z \sim 5$ . Lighter seeds ( $10^5 M_\odot$ ) in overdense regions evolve even faster, with specific realizations (e.g., the "2" realization initialized in a progenitor halo) reaching $M_\bullet/M_\star \sim O(10^{-4})$ by $z \sim 10$ , approaching the local scaling relation range.
The "Road to Normalcy":
The paper identifies a transient evolutionary phase where PBH-seeded systems resemble compact, BH-dominated sources (similar to observed LRDs) before evolving into extended galaxy-AGN systems.
- In the $10^7 M_\odot$ overdense case, the system transitions from a compact, BH-dominated state ( $M_\bullet/M_\star \sim 10$ , $R_{1/2} \approx 110 \text{ pc}$ at $z \approx 8.4$ ) to a more conventional galaxy ( $M_\bullet/M_\star \sim 0.2$ , $R_{1/2} \sim 300 \text{ pc} - 1 \text{ kpc}$ ) by $z \approx 4$ .
- This transition is driven primarily by rapid stellar assembly and spatial growth of the host, rather than substantial BH growth (which increases by only a factor of $\sim 1.4$ during this interval).
- Lighter seeds ( $10^5 M_\odot$ ) in overdense regions evolve even faster toward "normalcy," with $M_\bullet/M_\star$ dropping to $O(10^{-4})$ by $z \sim 10$ in specific configurations, effectively erasing the memory of the initial massive seed.
Metallicity and Morphology:
- Metallicity: Overdense systems exhibit sustained chemical enrichment, with halo metallicities rising from $Z \sim 10^{-3} Z_\odot$ to $\sim 0.1 Z_\odot$ , consistent with observations of LRDs and GN-z11. Isolated systems experience only brief enrichment followed by dilution, remaining extremely metal-poor.
- Morphology: The spatial distribution of the PBH relative to the stellar component varies. In some cases, the PBH remains offset from the dominant stellar cluster (resembling systems like Hebe), while in others, it is centrally embedded. The compact, LRD-like appearance is not universal; highly overdense realizations can form extended, clumpy hosts at earlier times.
Comparison with Direct Collapse Black Holes (DCBHs):
Unlike DCBH models, where the BH forms after baryonic structure formation has begun, PBHs are present prior to host galaxy formation. Consequently, PBH systems show a monotonic trend of decreasing $M_\bullet/M_\star$ as the host assembles, whereas DCBH systems exhibit more fluctuating evolutionary tracks dependent on merger events and radiative conditions.

Significance and Claims
The authors claim that their results provide a unified framework for understanding the diversity of early BH-galaxy configurations observed by JWST. The study suggests that:

Environmental Regulation: The initial seed properties (mass, formation channel) and the cosmological environment jointly shape early co-evolution.
Erosion of Initial Signatures: Subsequent environmental regulation can effectively erase the memory of the initial seeding channel. A system that was initially seeded by a massive PBH may evolve to appear indistinguishable from a conventional galaxy-AGN system by $z \sim 10$ if it resides in an overdense region, particularly for lighter seed masses.
Transient Phases: Compact, BH-dominated sources (LRDs) may naturally arise as a specific phase in the evolutionary pathway of PBH-seeded systems in overdense environments, rather than representing a unique, static class of objects.
Observational Implications: The persistence of extreme BH-to-stellar mass ratios is highly dependent on the environment. Detecting the "fingerprint" of a PBH seed likely requires observing systems at high redshift ( $z \gtrsim 10$ ) before host assembly drives them toward normalcy, or identifying specific geometric configurations (e.g., offset seeds) that survive longer.

The paper concludes that while PBHs offer a viable mechanism for early BH seeding, their observable signatures are transient and heavily modulated by the large-scale structure of the universe, necessitating a shift from searching for a single "PBH signature" to mapping the diverse evolutionary pathways these systems traverse.

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