🔭 astrophysics

Growth of Light Seed Black Holes in the Early Universe

This paper demonstrates through high-resolution cosmological simulations that light seed black holes, remnants of Population III stars, can rapidly grow to $\sim10^4 \mathrm{M}_{\odot}$ in the early Universe, providing a viable explanation for the supermassive black holes observed by JWST at redshifts $z > 8$ .

Original authors: Daxal H. Mehta, John A. Regan, Lewis Prole

Published 2026-01-22

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Daxal H. Mehta, John A. Regan, Lewis Prole

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

The Big Mystery: The "Giant" Problem

Imagine you walk into a forest and see a massive oak tree that is fully grown. You know trees take hundreds of years to get that big. But then, you look at the date on the calendar, and it says the forest was only planted one week ago.

This is the puzzle astronomers are facing with the James Webb Space Telescope (JWST). It has found "Supermassive Black Holes" (SMBHs)—the cosmic giants that sit at the centers of galaxies—that are already fully grown when the universe was very young (less than a billion years old).

The problem is: How do you grow a giant black hole so fast? The usual suspects are "Light Seed Black Holes" (LSBHs). These are the tiny remnants left behind when the very first stars in the universe (called Population III stars) die. They are like the acorns of the forest. The question is: Can an acorn grow into a giant oak in just a few weeks?

The Experiment: Building a Better Microscope

For a long time, computer simulations (which are like video games of the universe) said "No." They showed that these tiny black holes usually get stuck. They couldn't eat enough gas to grow big because the gas around them was too hot or too scattered.

However, the authors of this paper argue that the previous simulations were like looking at the forest with a blurry camera. They couldn't see the tiny details right next to the black hole.

To fix this, the team built a new, incredibly high-resolution simulation. Think of it as swapping a standard-definition TV for an 8K Ultra-HD screen. They zoomed in so close that they could see the immediate neighborhood of the black hole, resolving distances as small as 0.1 light-years (about the size of our solar system).

The Discovery: The "Feast"

With this new, super-sharp view, they found that the tiny black holes can grow, but only under very specific conditions.

The Direct Collapse Advantage: Not all first stars die the same way. Some explode violently (like a supernova), blowing away all the food (gas) the black hole needs. Others simply collapse straight into a black hole without exploding. The paper found that the black holes that grew the fastest were the ones that collapsed directly. They didn't blow their own buffet away.
The Super-Eating Spree: These lucky black holes didn't just eat slowly; they went on "binge-eating" sprees. They ate gas at rates far faster than the theoretical limit (called the Eddington limit). Imagine a human eating 1,000 burgers in an hour. That's what these black holes were doing.
The Result: In their simulations, these tiny seeds (starting at about the mass of a large star) managed to grow into "Intermediate Mass Black Holes" (about 10,000 times the mass of our Sun) in just a few million years.

The Obstacle: The "Feedback" Wall

The paper also tested what happens when the black hole starts fighting back. As a black hole eats, it gets hot and shoots out energy (radiation and heat). This is like a person eating so fast they start sweating and shouting, which scares away the other people at the table.

The team ran a simulation where they turned on this "feedback" (the shouting and sweating).

The Effect: The heat pushed the gas away, stopping the black hole from eating.
The Surprise: Even with this "shouting" feedback, some black holes still managed to grow huge. They would eat a bit, get stopped, wait for the gas to cool down and fall back in, and then eat again. It was a stop-and-go feast, but they still got big enough to be the ancestors of the giants we see today.

The Conclusion: A Bridge to the Giants

The paper concludes that we don't need to invent new, exotic physics to explain the giant black holes JWST found. We just needed to look closer.

If we have high enough resolution, we see that:

Tiny black holes (acorns) can grow into medium-sized black holes (saplings) very quickly.
This happens through short, intense bursts of eating.
Even when they try to stop themselves by heating up their food, they can still grow.

These "medium-sized" black holes are the perfect bridge. They are big enough that, if they survive into the later universe, they can easily grow into the Supermassive Black Holes we see today. The paper essentially says: "The acorns were there all along; we just needed a better microscope to see them growing."

Technical Summary: Growth of Light Seed Black Holes in the Early Universe

Problem Statement
Recent observations from the James Webb Space Telescope (JWST) have revealed supermassive black holes (SMBHs) with masses exceeding $10^6 M_\odot$ at redshifts $z > 8$ . These findings challenge existing models of early black hole formation, as standard growth mechanisms struggle to explain how black holes could reach such masses within the available cosmic time. While light seed black holes (LSBHs)—remnants of the first generation of Population III (PopIII) stars—are theoretically viable progenitors, previous large-scale cosmological simulations have consistently failed to demonstrate significant LSBH growth. This failure is attributed to insufficient resolution to resolve the Bondi-Hoyle-Littleton (BHL) accretion radius of light seeds and the inability to capture the dense, small-scale gas environments required for efficient, super-Eddington accretion in the presence of feedback.

Methodology
To address these limitations, the authors conducted a suite of high-resolution cosmological zoom-in simulations using the moving-mesh code Arepo. The study employed four distinct simulation realizations (L13, L14, L15, and L15 BHFB) with progressively increasing spatial and mass resolution, initialized at $z=127$ and evolved to $z \sim 18-21$ .

Resolution: The highest-resolution run (L15) achieved a minimum spatial resolution of 0.1 pc and a dark matter particle mass of 159 $M_\odot h^{-1}$ . This resolution is critical as it resolves the BHL radius of the smallest black holes (approx. 0.5 pc for a $100 M_\odot$ BH) and matches the mass scale of massive PopIII stars, allowing for robust modeling of gravitational dynamics and accretion.
Physics Models: The simulations include comprehensive sub-grid physics:
- Star Formation: Metal-free PopIII stars form in high-density, metal-poor regions with a top-heavy initial mass function (IMF) characterized by a mass of $20 M_\odot$ . PopII stars form in metal-enriched environments ( $Z > 10^{-4} Z_\odot$ ).
- Black Hole Seeding: PopIII stars transition to black holes via direct collapse or supernova (SNe) channels based on their Zero Age Main Sequence (ZAMS) mass.
- Accretion: BH growth is modeled using the Bondi-Hoyle-Lyttleton (BHL) method, modified to include the effects of gas vorticity and turbulence.
- Feedback: The simulations incorporate SNe feedback (momentum injection) and, in the L15 BHFB run, thermal feedback from BH accretion. A redshift-dependent Lyman-Werner background flux is also included to mimic external feedback.

Key Results
The simulations demonstrate that LSBHs can undergo rapid growth to masses of $\sim 10^4 M_\odot$ within a fully cosmological setting, provided the resolution is sufficient to resolve the accretion physics.

Growth Efficiency: In the highest-resolution runs (L15 and L15 BHFB), numerous BHs grew to exceed $10^4 M_\odot$ . Specifically, the most massive BH in the L15 BHFB simulation (which includes thermal feedback) grew from an initial $66 M_\odot$ to $\sim 1.4 \times 10^4 M_\odot$ by $z \approx 21.5$ .
Accretion Modes: Growth occurs primarily through gas accretion rather than mergers (only 8 mergers were observed across all simulations). The growth is characterized by short, intense bursts of super-Eddington accretion, with Eddington factors reaching up to $10^3$ for brief durations.
Impact of Feedback: While thermal feedback from BH accretion and SNe feedback from surrounding stars effectively evacuate gas and halt growth in many cases, the simulations show that LSBHs can still achieve significant growth. This occurs either before feedback fully clears the environment or when gas re-collapses onto the BH, triggering renewed accretion episodes.
Formation Channel: A distinct correlation was found between the growth outcome and the formation channel. All growing LSBHs originated from the direct collapse channel (PopIII stars with $40 < M < 140 M_\odot$ or $M > 260 M_\odot$ ). BHs formed via the SNe channel showed no significant growth, as the associated explosion likely clears the local gas reservoir.
Environmental Dependence: No strong correlation was found between BH growth and the macro-properties of the host halo (e.g., total halo mass). Instead, growth appears stochastic and driven by small-scale fluctuations in gas density and the specific formation channel. Growth is possible in both pristine metal-free environments and metal-enriched galaxies.

Significance and Claims
The authors claim that their results provide robust evidence that LSBHs can serve as the dominant progenitors for the SMBHs observed at later cosmic epochs. The study bridges the gap between idealized isolated simulations and large-volume cosmological models by demonstrating that:

Resolution is Paramount: The failure of previous cosmological simulations to grow LSBHs was likely due to insufficient resolution to capture the BHL radius and the resulting accretion dynamics.
Viability of Light Seeds: Despite the challenges posed by feedback, light seeds formed from PopIII stars can attain masses of $10^3 - 10^4 M_\odot$ in the early universe. These masses align with the seed masses typically assumed in lower-resolution simulations that successfully produce SMBHs at lower redshifts.
Implications for JWST and LISA: The findings offer a plausible formation pathway for the high-redshift SMBHs detected by JWST and suggest that the population of intermediate-mass black holes formed in this manner could be significant targets for future gravitational wave detections with LISA.

The paper concludes that while the simulations are limited in runtime and cannot trace the long-term evolution of these seeds into full SMBHs, the demonstrated early growth phase positions LSBHs as compelling candidates for the origin of the observed SMBH population.

The Big Mystery: The "Giant" Problem

The Experiment: Building a Better Microscope

The Discovery: The "Feast"

The Obstacle: The "Feedback" Wall

The Conclusion: A Bridge to the Giants

More like this