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SEEDZ: Rapid Galaxy Assembly as a Pathway to Supermassive Stars, Dense Stellar Environments and Massive Black Hole Seeds

The SEEDZ hydrodynamic simulations demonstrate that rapid galaxy assembly drives high gas inflow rates necessary for forming supermassive stars and massive black hole seeds, suggesting that a tiny fraction of these objects could explain the observed population of Little Red Dot galaxies.

Original authors: Lewis R. Prole, John A. Regan, Daxal Mehta, Devesh Nandal, Rüdiger Pakmor, Ricarda S. Beckmann, Michael Tremmel, Martin G. Haehnelt, Simon C. O. Glover, Ralf S. Klessen, John H. Wise, Sophie Koudmani

Published 2026-06-29

📖 5 min read🧠 Deep dive

CC BY 4.0

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

Imagine the early universe as a vast, dark construction site. For a long time, astronomers have been trying to figure out how the "supermassive black holes" (the giant monsters at the centers of galaxies) got so big so quickly. They are like skyscrapers that appeared fully grown in a matter of seconds, which shouldn't be possible if they started as tiny pebbles.

This paper, titled SEEDZ, is like a high-definition time-lapse movie of that construction site. The researchers used a powerful computer simulation to watch how early galaxies were built and to see if they could find the "seeds" that grew into these giant black holes.

Here is the story of what they found, explained simply:

1. The Two Ways to Build a Monster

The scientists were looking for two specific ways a "heavy seed" (a massive starting point for a black hole) could be born:

The "Supermassive Star" (SMS): Imagine a single, giant star that eats so much gas so fast that it swells up to become a monster before collapsing into a black hole. This requires a very specific, quiet environment with very little "dirt" (metals) and a huge, steady stream of gas flowing in.
The "Dense Stellar Cluster": Imagine a crowded dance floor where thousands of stars are packed tightly together. They bump into each other and merge, eventually forming a heavy object in the center. This happens in environments that are a bit "dirtier" (more metals) and more chaotic.

2. The "Feeding Frenzy" Requirement

To get a Supermassive Star, you need a massive pipeline of gas flowing into a tiny area. The researchers set a rule: if more than one sun's worth of gas flows into a 10-light-year-wide area every year, a heavy seed might form.

They found that the galaxies capable of doing this are the "fast growers."

The Analogy: Think of two houses being built. One house is built slowly, brick by brick. The other house is being built by a tornado that sucks up all the bricks and slams them together instantly. The "tornado" galaxies (the fast growers) are the only ones that can create the conditions needed for these heavy seeds.

3. The Timeline: Patience is Key

The simulation showed that these heavy seeds don't appear immediately.

First, the galaxy forms its very first stars (the "minihalo" stage).
Then, it takes about 200 million years of rapid growth and gas flowing inward before the conditions are right for a heavy seed to form.
It's like waiting for a river to flood a valley; you need the water level to rise high and fast before the dam breaks and the heavy seed is born.

4. The Big Surprise: It's Mostly the "Crowded Dance Floor"

The researchers expected to find lots of Supermassive Stars (the single giant stars). However, they found something different:

The Reality: Most of the heavy seeds (about 98%) formed in environments that were already "dirty" with metals (like dust and heavy elements).
The Result: In these dirty environments, the gas breaks apart into many smaller stars instead of one giant one. This leads to the "Dense Stellar Cluster" scenario, where stars crash into each other to build the seed.
The Rare Exception: Only a tiny fraction (about 13 out of 932) formed in the "clean," metal-free environments required for a Supermassive Star. Even then, only a few of those actually sustained the high feeding rate needed to become a true SMS.

5. Connecting to the "Little Red Dots"

Recently, the James Webb Space Telescope (JWST) has spotted strange, tiny, red galaxies called "Little Red Dots." Scientists think these might be the glowing hearts of these early black holes.

The paper does a quick math check:

The simulation predicts there are a lot of these potential "Supermassive Stars" in the universe (about 0.1 per cubic billion light-years).
The number of "Little Red Dots" we actually see is much lower.
The Conclusion: The researchers found that if only one in every 10,000 of these Supermassive Stars is bright enough for JWST to see, that perfectly explains the number of "Little Red Dots" we are finding. We don't need to see them all; just a tiny fraction is enough to match what we observe.

Summary

The paper tells us that the early universe was a busy construction zone. While we hoped to find giant, solitary stars collapsing into black holes, the simulation suggests that most of these heavy seeds were actually built by crowds of stars merging together in metal-rich environments. The "lonely giant star" path is real, but it's the rare exception, not the rule. However, even those rare giants are numerous enough to explain the mysterious red dots we see in the deep universe today.

Technical Summary: SEEDZ: Rapid Galaxy Assembly as a Pathway to Supermassive Stars, Dense Stellar Environments and Massive Black Hole Seeds

Problem Statement
The origin of supermassive black holes (SMBHs, $>10^6 M_\odot$ ) observed at high redshifts ( $z \sim 10$ ) remains a subject of debate. Current models generally posit two pathways: the rapid growth of remnants from the first Population III stars (initial masses of tens of $M_\odot$ ) or the formation of "heavy seeds" with initial masses exceeding $10^3 M_\odot$ . Heavy seeds may arise from mergers in dense stellar clusters, quasi-stars, or rapidly accreting supermassive stars (SMSs). A critical requirement for SMS formation is sustained high mass inflow rates ( $\dot{M} \gtrsim 0.02 M_\odot \text{yr}^{-1}$ ) in low-metallicity environments to prevent fragmentation. However, the specific assembly histories and environmental conditions that facilitate these high inflow rates in early galaxies are not fully understood. Furthermore, recent JWST observations of compact, red galaxies known as "Little Red Dots" (LRDs) suggest they may host these heavy seeds or SMSs, yet the connection between galaxy assembly, seed formation, and LRD observables requires numerical investigation.

Methodology
This study utilizes the SEEDZ hydrodynamic simulation framework, specifically the "HighRes" suite, which offers an order-of-magnitude improvement in resolution over previous calibration runs. The simulations employ the AREPO2 code with a parent box of $40 \text{ cMpc } h^{-1}$ and a high-resolution zoom-in region ( $1.25 \times 2.02 \times 1.21 \text{ cMpc}^3$ ) initialized at $z=127$ . The resolution achieves a minimum gas cell size of $5 \text{ pc } h^{-1}$ ( $\sim 0.67$ physical pc at $z=10$ ) and dark matter particle masses of $1.0 \times 10^4 M_\odot h^{-1}$ .

The study employs a subgrid model for heavy seed formation based on mass inflow criteria rather than resolving the sub-parsec physics directly. A heavy seed black hole forms when the radial mass flux within a $10 \text{ pc}$ sphere surrounding a gravitationally unstable cell exceeds $1 M_\odot \text{yr}^{-1}$ . The seed mass is sampled from a power law ( $5 \times 10^3 - 10^5 M_\odot$ ). The model distinguishes between Population III star formation (low metallicity, low inflow), Population II clusters (higher metallicity, low inflow), and heavy seeds (high inflow). Notably, AGN feedback is disabled in this dataset to preserve the environmental conditions of newly formed seeds for analysis.

To interpret the heavy seeds, the authors apply physical criteria derived from stellar evolution models:

SMS Formation: Requires metallicity $Z < 10^{-2} Z_\odot$ and sustained inflow $\dot{M} > 0.02 M_\odot \text{yr}^{-1}$ over a $\sim 2 \text{ Myr}$ lifetime.
Dense Stellar Cluster Formation: Occurs in metal-enriched environments ( $Z > 10^{-2} Z_\odot$ ) where fragmentation leads to cluster formation rather than monolithic SMS collapse.

Key Results

Halo Assembly Histories: Halos that host heavy seeds exhibit significantly accelerated growth rates compared to non-seed-forming halos. While both groups form their first stars in minihalos ( $\sim 10^6 M_\odot$ ), seed-forming halos grow rapidly in the subsequent 100 Myr, typically reaching masses $>10^8 M_\odot$ . Heavy seeds form approximately 200 Myr after the first star formation in these halos.
Inflow Rates and Density: At the time of heavy seed formation, these halos sustain mass inflow rates exceeding $0.1 M_\odot \text{yr}^{-1}$ into the inner $1 \text{ pc}$ , cascading from virial radii of a few hundred parsecs. This maintains high central densities ( $10^4 - 10^5 \text{ cm}^{-3}$ ) within the central $10 - 100 \text{ pc}$ .
Metallicity Dependence: The vast majority of heavy seeds (919 out of 932) form in metal-enriched environments ( $Z > 10^{-2} Z_\odot$ ), often near-solar metallicity by $z=10$ . Only 13 heavy seeds form in regions with $Z < 10^{-2} Z_\odot$ , and only 3 are truly metal-free. This suggests that by the time heavy seeds form, the host halos have already been enriched by prior generations of stars.
SMS vs. Cluster Candidates: Applying the SMS criteria ( $Z < 10^{-2} Z_\odot$ and $\dot{M} > 0.02 M_\odot \text{yr}^{-1}$ ), the simulation yields a number density of SMSs of $0.1 \text{ cMpc}^{-3}$ . In contrast, the total heavy seed number density is $\sim 100 \text{ cMpc}^{-3}$ , dominated by high-metallicity candidates likely forming via dense stellar cluster collisions.
LRD Connection: The calculated SMS number density ( $0.1 \text{ cMpc}^{-3}$ ) implies that only a fraction of $\sim 10^{-4}$ of these SMSs would need to be visible to JWST to account for the observed population of Little Red Dot galaxies (estimated at $n_{LRD} \sim 10^{-5} \text{ cMpc}^{-3}$ at lower redshifts).

Significance and Claims
The paper claims that the SEEDZ framework successfully identifies the environmental prerequisites for heavy seed formation, demonstrating that rapid halo assembly is the primary driver for the high inflow rates necessary for these objects. The study concludes that while SMS formation is a viable pathway, it is likely a minority channel, restricted to low-metallicity pockets in the early universe. The dominant pathway for heavy seed formation in the simulated epoch appears to be the formation of massive black holes within dense, metal-enriched stellar clusters.

The authors posit that the conditions required for SMS formation (high inflow, low metallicity) are met in specific early galaxies, providing a potential explanation for the spectral features of LRDs. However, they emphasize that distinguishing between SMS progenitors and dense stellar cluster progenitors at the simulation's resolution is challenging. The work provides a statistical basis for the rarity of SMSs compared to other heavy seed formation mechanisms and suggests that the observed LRD population could be consistent with a small, visible fraction of the simulated SMS population.