🔭 astrophysics

The SEEDZ Simulations: Methodology and First Results on Massive Black Hole Seeding and Early Galaxy Growth

The SEEDZ simulations introduce a comprehensive suite of cosmological hydrodynamic models that demonstrate massive black holes initially grow faster than their host galaxies at high redshifts ( $z=15$ ), resulting in over-massive black holes that deviate from the established $z=0$ black hole-galaxy relationships.

Original authors: Lewis R. Prole, John A. Regan, Daxal Mehta, Rudiger Pakmor, Sophie Koudmani, Martin A. Bourne, Simon C. O. Glover, John H. Wise, Ralf S. Klessen, Michael Tremmel, Debora Sijacki, Ricarda S. Beckmann

Published 2026-03-19

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Lewis R. Prole, John A. Regan, Daxal Mehta, Rudiger Pakmor, Sophie Koudmani, Martin A. Bourne, Simon C. O. Glover, John H. Wise, Ralf S. Klessen, Michael Tremmel, Debora Sijacki, Ricarda S. Beckmann, Martin G. Haehnelt, John Brennan, Pelle van de Bor, Paul C. Clark

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 Picture: A Cosmic Detective Story

Imagine the universe as a giant, dark forest. For a long time, astronomers thought the first "trees" (galaxies) grew slowly, and the "monsters" living inside them (massive black holes) were just tiny babies that grew up very slowly over billions of years.

But recently, the James Webb Space Telescope (JWST) has been taking high-definition photos of this ancient forest. And guess what? The photos show massive monsters already fully grown in the very first few hundred million years of the universe. It's like walking into a forest and finding a fully grown oak tree that was supposed to be a sapling.

This is a problem. How did these black holes get so big, so fast?

Enter the SEEDZ simulations. Think of SEEDZ as a super-powered, virtual time machine. The scientists built a digital universe to test different theories on how these "black hole seeds" were planted and how they grew into the giants we see today.

The Two Types of Seeds

The paper explores two main ways these black holes might have started their lives:

The "Light" Seed (The Tiny Sprout):
- The Theory: These come from the death of the very first stars (Population III stars). When these stars explode, they leave behind a tiny black hole, maybe the size of a few suns.
- The Problem: In the SEEDZ simulation, these tiny seeds are like trying to grow a giant tree from a single grain of sand. They struggle to eat enough gas to get big quickly. In this specific run of the simulation, they barely grew at all because the "resolution" (the detail level) wasn't high enough to see them eating properly.
The "Heavy" Seed (The Giant Acorn):
- The Theory: Instead of a tiny sprout, maybe the universe sometimes drops a giant acorn. This happens when a massive cloud of gas collapses so fast it skips the "star" phase and turns directly into a black hole, or when a cluster of stars crashes together. These start off huge (thousands of times the mass of our Sun).
- The Result: In the SEEDZ simulation, these heavy seeds are the stars of the show. They start big and grow even bigger very quickly.

How the Simulation Works: The "Smart" Universe

The scientists used a supercomputer code called AREPO. Imagine this code as a digital chef cooking a cosmic stew.

The Ingredients: Dark matter, gas, and radiation.
The Recipe (The "SmartStar" Model): The computer doesn't just guess when a star forms. It looks at the pressure and density of the gas. If a cloud gets too heavy and unstable, the computer says, "Okay, time to make a star or a black hole."
The Feedback Loop (The Thermostat): This is crucial. When a black hole eats gas, it gets hot and shoots out energy (like a cosmic blowtorch). This heat pushes away the surrounding gas, stopping the black hole from eating more. It's like a thermostat: if the room gets too hot, the heater turns off. The paper found that this "blowtorch" effect often stops the black holes from growing too fast, but for a short time, they can eat at "super-speed" (super-Eddington rates).

The Big Discoveries

After running the simulation up to a time when the universe was very young (about 300 million years old, or redshift $z=15$ ), here is what they found:

1. The "Over-Massive" Baby Black Holes
In our local neighborhood today (the modern universe), black holes and their host galaxies have a perfect relationship. If the galaxy is big, the black hole is big. It's like a perfect dance partner.

The Discovery: In the early universe, this dance was messy. The black holes grew much faster than their host galaxies.
The Analogy: Imagine a toddler (the black hole) growing to be 6 feet tall while their parents (the galaxy) are still toddlers. The black holes were "over-massive" compared to the galaxies holding them. The paper suggests that the perfect relationship we see today only forms later, once the galaxies have had time to catch up and mature.

2. The "Little Red Dots"
Astronomers have found strange, tiny, red galaxies that seem to hide massive black holes.

The Discovery: The SEEDZ simulation successfully created these kinds of systems. The heavy seeds grew so fast that they became massive while the galaxy around them was still small and compact. This supports the idea that these "Little Red Dots" are indeed early galaxies with hungry, overgrown black holes in the center.

3. Growth Speed
The heavy seed black holes didn't just grow; they gorged themselves. For a short period (1 to 15 million years), they ate gas at rates far exceeding the theoretical speed limit (the Eddington limit). They grew from a few thousand suns to a million suns in a cosmic blink of an eye.

The Limitations (The "Blurry Camera")

The authors are honest about the limits of their simulation.

Resolution: Think of the simulation like a photo. It's very high quality, but not perfect quality. They can see the "Heavy Seeds" clearly, but the "Light Seeds" are too small to track properly in this version.
Mergers: In the real universe, black holes might grow by crashing into each other (merging). In this simulation, the "camera" wasn't sharp enough to see those crashes clearly, so the black holes mostly grew by eating gas, not by merging.

The Conclusion: What's Next?

The SEEDZ team has built a solid foundation. They've shown that heavy seeds are a very plausible explanation for the giant black holes JWST is finding.

The Plan: They are going to keep running the simulation until the universe is a bit older ( $z=10$ ).
The Goal: They will compare their digital results directly with the real photos from JWST. If their "digital universe" matches the "real universe" photos, it will prove that these heavy seeds are indeed how our universe's monsters were born.

In a nutshell: The universe didn't start with a perfect balance. It started with chaos, where massive black holes grew up too fast, leaving their host galaxies in the dust. Over time, the galaxies caught up, and today, they finally dance in step. The SEEDZ simulations are the story of that chaotic childhood.

1. Problem Statement

The paper addresses the growing tension between recent observations from the James Webb Space Telescope (JWST) and theoretical models of early universe evolution. JWST has detected a surprisingly high number density of massive black holes (MBHs, $10^6 - 10^8 M_\odot$ ) and compact "Little Red Dot" (LRD) galaxies at high redshifts ( $z > 6$ ).

The Challenge: Standard cosmological models struggle to explain how black holes could grow to such massive sizes so quickly (within the first billion years of the universe) starting from "light" seeds (remnants of Population III stars, $\sim 10-100 M_\odot$ ).
The Gap: Existing large-scale cosmological simulations often rely on simplified seeding prescriptions (e.g., placing a fixed-mass black hole in every halo) or lack the resolution to model the complex interplay between metal-free star formation, heavy seed formation, and feedback mechanisms. There is a need for a self-consistent framework that models both "light" and "heavy" seeding channels to determine which pathways can reproduce the observed high-redshift population.

2. Methodology

The authors introduce the SEEDZ simulation suite, a set of cosmological hydrodynamic simulations designed to model the formation and growth of the first black holes.

Code & Physics: The simulations utilize AREPO2, a state-of-the-art moving mesh code. The fluid dynamics are solved using a finite-volume scheme on an unstructured Voronoi mesh.
Seeding Framework (SmartStar): The core innovation is the SmartStar sink particle framework, which dynamically determines the outcome of gravitational collapse based on local conditions:
- Light Seeds: Formed from the end-of-life of Population III (PopIII) stars. PopIII stars form in metal-free ( $Z < 10^{-4} Z_\odot$ ) environments with sufficient $H_2$ . They evolve via supernovae (SNe) or direct collapse into black holes ( $\sim 10-100 M_\odot$ ).
- Heavy Seeds: Formed stochastically in regions experiencing rapid gravitational collapse with mass inflow rates exceeding $1 M_\odot \text{yr}^{-1}$ . This criteria is agnostic to metallicity or $H_2$ fraction, allowing for the formation of heavy seeds ( $10^3 - 10^5 M_\odot$ ) via supermassive star (SMS) collapse or dense stellar cluster collapse.
- Population II (PopII): Once metal enrichment occurs ( $Z > 10^{-4} Z_\odot$ ), the model switches to PopII star cluster formation.
Feedback & Accretion:
- Accretion: Modeled via Bondi-Hoyle-Lyttleton accretion, adjusted for vorticity and sub-grid resolution (kernel radius).
- Feedback: Includes thermal feedback from SNe (momentum injection) and black hole accretion. The model allows for super-Eddington accretion, calculating radiative efficiency based on black hole spin and accretion rate.
- Dynamical Friction: A sub-grid model is implemented to correct for the artificial suppression of dynamical friction due to gravitational softening, aiding the migration of black holes to halo centers.
Simulation Setup:
- Parent Box: A $40 \text{ cMpc } h^{-1}$ dark-matter-only box evolved to $z=10$ .
- Zoom Regions: Three regions were selected for high-resolution hydrodynamic re-simulation:
  1. Rarepeak: Centered on the most massive halo ( $M \sim 1.8 \times 10^{11} M_\odot$ at $z=10$ ).
  2. Normal1 & Normal2: Centered on typical halos ( $M \sim 10^{10} M_\odot$ ).
- Resolution: Minimum cell size of $\sim 6$ physical pc at $z=15$ ; dark matter particle mass $\sim 7 \times 10^4 M_\odot$ .

3. Key Contributions

Unified Seeding Model: SEEDZ is one of the first suites to self-consistently model both light and heavy seed formation channels within a single cosmological framework, rather than imposing them as separate initial conditions.
Heavy Seed Criteria: The model introduces a robust, metallicity-independent criterion for heavy seed formation based on mass inflow rates, capturing the physics of SMS and dense cluster collapse without relying on specific sub-resolution prescriptions.
High-Redshift Evolution: The simulations track the evolution from $z \approx 30$ down to $z=15$ (with plans to reach $z=10$ ), bridging the gap between the first stars and the epoch of reionization.

4. Key Results (at $z=15$ )

Black Hole Growth:
- Heavy Seeds: Black holes formed from heavy seeds ( $10^3 - 10^5 M_\odot$ ) grow rapidly, reaching masses up to $10^6 M_\odot$ by $z=15$ .
- Super-Eddington Accretion: Heavy seeds accrete at rates exceeding the Eddington limit for $\sim 1-15$ Myr after formation.
- Light Seeds: Due to current resolution limits, light seeds (PopIII remnants) do not grow significantly in this suite; they remain near their birth mass.
Over-Massive Black Holes:
- Black holes initially grow faster than their host galaxies.
- The resulting $M_{BH} - M_*$ (black hole mass vs. stellar mass) ratios are significantly higher than local ( $z=0$ ) scaling relations, with some systems showing $M_{BH}/M_* \sim 10^{-2}$ .
- This suggests the fundamental $M_{BH} - M_*$ relationship observed today has not yet been established in these early, immature galaxies.
Environmental Dependence:
- While the heavy seed formation criteria is metallicity-agnostic, the majority of heavy seeds that grow to $>10^5 M_\odot$ form in low-metallicity environments ( $Z \ll 10^{-4} Z_\odot$ ).
- Some massive black holes remain embedded in near-pristine gas environments at $z=15$ , consistent with recent JWST observations of SMBHs in metal-poor galaxies.
Merger vs. Accretion:
- Growth is dominated by accretion ( $>99\%$ ).
- Merger-driven growth is negligible ( $<1\%$ ), likely an artifact of the current resolution not fully resolving the dynamics of black hole binaries.
Number Densities:
- The simulations produce a number density of massive black holes ( $>10^5 M_\odot$ ) of $\sim 10^{-2} \text{ cMpc}^{-3} h^3$ .
- This is consistent with some numerical estimates but higher than models relying solely on Lyman-Werner (LW) flux suppression.

5. Significance and Future Work

Interpretation of JWST Data: The results provide a theoretical basis for the "over-massive" black holes and compact LRDs observed by JWST. The simulations suggest that the discrepancy between high-redshift observations and local scaling laws is a natural consequence of early, rapid black hole growth preceding galaxy maturation.
Validation: The paper sets the stage for a comprehensive comparison with JWST data at $z=10$ (planned for a future publication), specifically targeting black hole number densities and scaling relations.
Limitations & Next Steps:
- The current resolution is insufficient to accurately model the growth of light seeds or the dynamics of low-mass heavy seeds.
- Future iterations will increase mass and spatial resolution by factors of 8 and 4, respectively, to better capture light seed growth and merger rates.
- The suite will be extended to $z=10$ to directly confront observational constraints.

In summary, the SEEDZ simulations demonstrate that heavy seed formation via rapid mass inflow is a viable and efficient pathway to producing the massive black holes observed in the early universe, naturally leading to a population of over-massive black holes that precedes the establishment of local galaxy-black hole scaling relations.