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Gas Accretion versus BH Merger driven Growth Modes of Supermassive Black Holes and Implications for the Little Red Dots

Through cosmological hydrodynamic simulations, this study demonstrates that supermassive black holes initially grow via mergers before undergoing rapid phases of gas accretion that drive them toward the $M_{\rm BH}-M_{\star}$ relation, thereby providing a physical explanation for the distinct evolutionary paths of normal and overmassive black holes observed by JWST as "Little Red Dots."

Original authors: Paramita Barai

Published 2026-05-06

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CC BY 4.0

Original authors: Paramita Barai

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 universe as a vast, cosmic construction site. For decades, astronomers have tried to figure out how the "bosses" of this construction site—the supermassive black holes (SMBHs)—became so enormous, reaching weights billions of times greater than our Sun.

This article is like a detailed time-lapse film created by a computer simulation. The author, Paramita Barai, built a virtual universe (a box with a diameter of 50 million light-years) and observed how these black holes grew from tiny seeds into giants over 13 billion years. Here is the story of what she found, explained simply.

1. The two ways black holes grow

The study discovered that black holes do not grow in just one way. They have two distinct "modes" of eating, like a person switching from snacking to a huge feast.

The "snack" phase (mergers): When black holes are young and small (starting as seeds of about 100,000 Suns), they grow mainly through mergers. Imagine two small black holes colliding and sticking together like two droplets of mercury. This happens frequently in the early universe. It is a slow, steady way to go from a small seed to a medium-sized black hole (about 10 million Suns).
The "feast" phase (gas accretion): Once they reach this medium size, something changes. They stop merely colliding and begin eating gas from their host galaxies. This is the "feast." For a short, intense period (about 600 to 700 million years), these black holes gulp down gas at the maximum speed allowed by physics (the "Eddington rate"). In this short window, they grow 100 to 1,000 times larger, jumping from 10 million Suns to billions of Suns.

2. The problem of the "full stomach"

Why don't they keep growing forever? The article explains that black holes eventually run out of food.

Imagine the black hole as a very hungry guest at a party. When it starts eating gas at this "feast" speed, it consumes so much that it clears the entire buffet table (the center of the galaxy). Once the gas is gone, the black hole reaches a "full stomach" limit. It stops growing. The study shows that due to this self-inflicted starvation, even the largest black holes in the simulation never grow larger than about 10 billion Suns. They hit an upper limit because they have exhausted all available fuel.

3. The mystery of the "little red dots"

Recently, the James Webb Space Telescope (JWST) discovered strange, tiny, red galaxies in the early universe, called "Little Red Dots." In them, astronomers found black holes that were surprisingly heavy for their size.

The article uses its simulation to solve this mystery:

The "normal" black holes: Some of these early black holes had the "normal" size. The study suggests that these grew mainly through the "snack" method (merging with other black holes) rather than by eating gas.
The "overmassive" black holes: Those that were too large for their galaxies? These are the ones that held the "feast." They experienced a phase of extremely rapid gas consumption that made them grow huge very quickly before the gas ran out.

4. The role of "feedback"

The simulation also tested what happens when the black hole becomes too aggressive. When a black hole eats, it doesn't just swallow; it also expels energy (feedback).

Imagine the black hole as a chef who, while cooking, accidentally blows the kitchen away with a strong wind. This wind pushes the gas away. The study found that this "wind" is actually the most important factor. It not only stops the growth of the black hole; it also prevents the galaxy from forming new stars because it blows the fuel for star formation (gas) out of the house.

Summary

In short, this article tells us that supermassive black holes are like marathon runners who go for a short, intense sprint.

They start small and grow slowly through mergers with neighbors.
Then they go on a short, massive eating binge with gas, which makes them huge in less than a billion years.
They eventually consume all the gas in their neighborhood, which prevents them from growing further.
This explains why some early black holes are "overweight" (they had a big feast) and why there is a limit to how large they can become (they ran out of food).

The study confirms that while black holes and their host galaxies grow together, the black hole's own hunger ultimately limits both its own size and the size of the galaxy in which it lives.

Technical Summary: Gas Accretion versus BH Merger as Growth Modes of Supermassive Black Holes and Implications for "Little Red Dots"

Problem Statement
The formation and growth mechanisms of supermassive black holes (SMBHs) remain a subject of unresolved research, particularly regarding how they reached masses of $\ge 10^9 M_\odot$ within the first billion years of the universe ( $z \sim 6$ ). Although various scenarios exist – ranging from rapid super-Eddington accretion onto stellar seeds to the formation of direct-collapse seeds with large masses ( $10^5 M_\odot$ ) or mergers of intermediate-mass black holes (IMBHs) – the specific pathways remain unclear. Recent observations by the James Webb Space Telescope (JWST) have further complicated the picture by revealing a population of high-redshift ( $z \sim 4-12$ ) "Little Red Dots" containing SMBHs that are either "overmassive" relative to the stellar mass of their host galaxies or possess "normal" masses consistent with local scaling relations. Distinguishing between gas accretion and black-hole-merger (BH) driven growth modes is crucial for understanding the coevolution of SMBHs and their host galaxies as well as for explaining these diverse high-redshift populations.

Methodology
The study employs cosmological hydrodynamic simulations of $(50 \text{ Mpc})^3$ comoving volumes using a modified version of the GADGET-3 code, which utilizes Tree-PM and Smoothed Particle Hydrodynamics (SPH) methods. The simulations evolve from redshift $z=100$ to $z=0$ within a flat $\Lambda$ CDM cosmology model.

Resolution: The standard resolution uses $256^3$ dark matter and $256^3$ gas particles, with a gas particle mass of $4.61 \times 10^7 M_\odot$ and a gravitational softening length of $3.3 \text{ kpc}$ (comoving).
Physics: The model includes metal-line cooling, a time-dependent photoionizing background, a multiphase sub-resolution star formation (SF) model, and chemical enrichment tracking 11 atomic species. Supernova (SN) feedback is implemented kinetically with a mass-loading parameter $\eta=2$ and a wind velocity $v_{SN}=350 \text{ km/s}$ .
Black Hole Seeding and Growth: BHs are seeded in massive halos ( $M_{halo} > 10^{10} M_\odot$ , $M_* > 5 \times 10^7 M_\odot$ ) with initial masses of $10^4$ , $10^5$ , or $10^6 M_\odot$ . Growth occurs via Bondi-Hoyle-Lyttleton accretion (limited by the Eddington rate) and BH mergers when particles reside within the softening length and the relative velocity is subsonic.
Feedback: Kinetic AGN feedback is modeled by transferring momentum to gas within a bi-conical volume. The feedback efficiency ( $\epsilon_f$ ) and wind kick velocity ( $v_w$ ) are varied across simulation runs to test their influence on the $M_{BH} - M_*$ relation.
Simulation Runs: A series of runs (Table 1) varies the seed mass ( $10^4-10^6 M_\odot$ ), feedback efficiency ($0, 0.1, 1$), and kick velocity ( $1000, 5000, 10000 \text{ km/s}$ ), supplemented by control runs without BHs or without BH feedback.

Main Results

Two-Phase Growth Mechanism: The simulations reveal a distinct two-stage growth history for SMBHs.
- Merger-Dominated Phase: The initial growth of $10^5 M_\odot$ seeds to $\sim 10^7 M_\odot$ occurs primarily through BH mergers. During this phase, gas accretion is low and strongly sub-Eddington ( $\dot{M}_{BH}/\dot{M}_{Edd} < 0.001$ ).
- Accretion-Dominated Phase: Once BHs reach $\sim 10^7 M_\odot$ , they undergo a rapid growth burst driven by efficient gas accretion. This phase lasts approximately $600-700 \text{ Myr}$ , during which the Eddington ratio reaches unity. In this window, the BH mass increases by factors of $10^2 - 10^3$ , growing from $10^7 M_\odot$ to $(10^9 - 10^{10}) M_\odot$ .
Growth Cessation: Following this rapid accretion phase, SMBH growth halts. The simulations show that SMBHs do not exceed $\sim 10^{10} M_\odot$ by $z=0$ . This decline is attributed to the depletion of gas in galaxy centers caused by AGN feedback, which self-regulates further growth.
$M_{BH} - M_*$ Coevolution: Simulated galaxies initially lie below the local $M_{BH} - M_*$ correlation. Over cosmic time, they evolve upward toward the relation. The slope of the correlation remains relatively constant, while the intercept evolves. The standard run ( $10^5 M_\odot$ seeds, $\epsilon_f=0.1$ , $v_w=5000 \text{ km/s}$ ) best reproduces the local $z=0$ observational relation (Marconi & Hunt 2003).
Parameter Sensitivity: The BH seed mass has the most significant influence on the final $M_{BH} - M_*$ relation; higher seeds ( $10^6 M_\odot$ ) lead to overmassive BHs, while lower seeds ( $10^4 M_\odot$ ) lead to undermassive BHs. Feedback efficiency and kick velocity have a comparatively lesser influence.
Star Formation: AGN feedback significantly suppresses star formation rates (SFR) at $z \le 4$ and reduces the global star formation rate density (SFRD) by factors of $1.3 - 15$ compared to simulations without BH feedback. Gas removal via direct accretion plays a subordinate role in suppression; the primary mechanism is feedback-induced heating and ejection.

Implications for Little Red Dots
The authors apply these growth modes to interpret JWST observations of high-redshift "Little Red Dots." They propose a dichotomy in formation channels:

Normal-Mass SMBHs: These objects likely underwent a predominantly BH-merger-driven evolution, consistent with the merger-dominated phase observed in the simulations.
Overmassive SMBHs: These objects likely experienced periods of Eddington-limited or super-Eddington bursts of efficient gas accretion, similar to the rapid growth phase identified in the simulations. The high nuclear gas densities in early-universe galaxies enabled this efficient accretion, resulting in BHs that appear overmassive relative to the stellar mass of their host galaxies.

Significance
The work provides a physical framework for distinguishing between merger-driven and accretion-driven growth regimes and suggests that the diverse populations of high-redshift SMBHs (normal vs. overmassive) observed by JWST may be attributable to different dominant growth pathways. The study demonstrates that AGN feedback is crucial for the self-regulation of SMBH growth by preventing them from exceeding $\sim 10^{10} M_\odot$ through the depletion of central gas reservoirs, and that it successfully reproduces the observed $M_{BH} - M_*$ scaling relations as well as the star formation histories of galaxies over cosmic time.

1. The two ways black holes grow

2. The problem of the "full stomach"

3. The mystery of the "little red dots"

4. The role of "feedback"

Summary

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