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On the unique evolutionary mechanisms of massive quiescent galaxies in the epoch of reionisation

This study utilizes the Thesan cosmological simulation to reveal that massive quiescent galaxies at $z=5.5$ form rapidly in dense environments through smooth accretion and prolonged black hole growth driven by a numerical suppression of AGN feedback, offering critical insights into the environmental conditions conducive to early galaxy quenching.

Original authors: Harry George Chittenden, Karl Glazebrook, Themiya Nanayakkara, Lalitwadee Kawinwanichakij, Claudia Lagos, Lucas Kimmig, Rhea-Silvia Remus

Published 2026-03-25

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CC BY 4.0

Original authors: Harry George Chittenden, Karl Glazebrook, Themiya Nanayakkara, Lalitwadee Kawinwanichakij, Claudia Lagos, Lucas Kimmig, Rhea-Silvia Remus

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 "Glitch" That Taught Us Something New

Imagine you are watching a movie about how galaxies grow up. Usually, these movies follow a strict script: a galaxy gathers gas, stars are born, and eventually, a giant monster in the center (a Supermassive Black Hole) wakes up, blows a giant "fan" (feedback) that pushes all the gas away, and the galaxy stops making stars. It grows old and "quiescent" (quiet).

However, astronomers using the James Webb Space Telescope (JWST) have found something weird. They are seeing massive, old-looking galaxies that stopped making stars way too early in the universe's history (when the universe was only about 1 billion years old). Standard computer models couldn't explain how these "early retirees" existed.

Then, the authors of this paper looked at a specific computer simulation called Thesan. They found nine massive, quiet galaxies there too. But here is the twist: The simulation had a bug.

The "Glitch" in the Matrix

In the Thesan simulation, a coding error accidentally turned down the volume on the Black Hole's "fan" (AGN feedback) by a factor of 25. At the same time, it accidentally turned up the volume on how much food (gas) the Black Hole could eat.

Think of it like a car engine with a broken governor.

Normal Universe: The engine has a governor that limits speed. If the car goes too fast, the governor cuts the fuel to keep it safe.
Thesan Universe (The Glitch): The governor is broken. The engine revs up to 500% capacity, eating fuel at a massive rate, but the brakes (feedback) are barely working.

Because the brakes were so weak, the Black Holes in Thesan grew to be giants very quickly. They ate so much that they eventually became so massive that they finally managed to shut down the galaxy's star formation, but only after they had grown to enormous sizes.

The Discovery: How the "Early Retirees" Were Made

The authors realized this "glitch" actually created a perfect laboratory to study a specific question: What happens if Black Holes grow unchecked in the early universe?

They found that these massive, quiet galaxies (MQGs) formed through a very specific recipe:

The Neighborhood Matters: These galaxies didn't form in empty space. They were born in the "cosmic city centers"—dense clusters of matter where everything is packed tight.
Smooth Accretion, Not Brawls: Usually, we think galaxies grow by smashing into each other (mergers). But these galaxies grew by smoothly soaking up gas from their dense surroundings. It was like a sponge soaking up water, rather than a car crash.
The Black Hole Feast: Because the environment was so dense and the "brakes" were broken, the central Black Hole ate continuously. It grew so fast and so big that it eventually became heavy enough to blow the gas away, killing the star formation.
The Aftermath: Even after the galaxy stopped making stars, its neighborhood (the halo) kept growing. It was like a quiet house sitting in a neighborhood that kept getting bigger and more crowded around it.

The "Two Types" of Quiet Galaxies

The paper splits these galaxies into two groups based on how long they've been quiet:

The "Recently Retired" (RQGs): These just stopped making stars. They are still in the process of being shut down.
The "Long-Term Retirees" (LQGs): These have been quiet for a long time. They are the most extreme examples. They live in the densest environments and have the biggest Black Holes.

The authors used a fancy statistical tool (Principal Component Analysis) to sort through thousands of galaxies and found that the "Long-Term Retirees" are distinct because they live in the most crowded neighborhoods and grew the fastest.

Why This Matters (Even with a Bug)

You might ask, "If the simulation had a bug, why does it matter?"

The authors argue that while the exact numbers in Thesan are wrong because of the bug, the physics it revealed might be real.

JWST Reality: We are seeing Black Holes in the real universe that are way too big for their host galaxies. This suggests that in the early universe, Black Holes did grow very fast, perhaps because the "brakes" (feedback) weren't working as well as they do today.
The Lesson: The Thesan simulation accidentally showed us a scenario where Black Holes grow unchecked until they are massive enough to shut down their galaxies. This helps explain how those "early retiree" galaxies found by JWST could exist.

The Takeaway

Think of the early universe as a chaotic construction site.

Standard Theory: The Black Hole is a strict foreman who stops construction (star formation) as soon as the building gets too big.
Thesan's "Glitch" Theory: The foreman is asleep. The construction crew (gas) keeps building the Black Hole until it's a skyscraper. Only when the skyscraper is massive does the foreman finally wake up, blow a whistle, and stop the whole construction site.

This paper suggests that maybe, in the very early days of the universe, the "foreman" was often asleep or weak, allowing Black Holes to grow huge before they finally shut down their galaxies. This helps us understand the strange, massive, quiet galaxies we are now seeing through our most powerful telescopes.

1. Problem Statement

Recent observations from the James Webb Space Telescope (JWST) have revealed an unexpectedly high abundance of massive quiescent galaxies (MQGs) at high redshifts ( $z \gtrsim 5$ ), some of which have already quenched star formation by $z \sim 11$ . Standard cosmological hydrodynamical simulations (e.g., IllustrisTNG, Eagle, Magneticum) struggle to reproduce this population, typically predicting that such massive galaxies are still actively forming stars during the epoch of reionisation.

The core challenge is understanding the physical mechanisms that allow massive galaxies to assemble rapidly and quench early. While some simulations (like Magneticum or Colibre) produce MQGs, they often rely on specific feedback calibrations or occur in different environmental contexts. The authors investigate the Thesan simulation suite, which incorporates explicit radiative transfer, to understand the evolutionary pathways of these rare objects. Notably, the authors identify a numerical issue in the public Thesan release that inadvertently creates a regime of suppressed AGN feedback, offering a unique "numerical experiment" to test the limits of galaxy-black hole co-evolution under weak coupling.

2. Methodology

The study utilizes the Thesan-1 simulation, a $95.5 \, \text{cMpc}^3$ volume cosmohydrodynamical simulation based on the Arepo moving-mesh code with on-the-fly radiative transfer (Arepo-RT).

Sample Selection: The authors identify nine massive central galaxies ( $M_* > 10^{10} M_\odot$ ) with specific star formation rates (sSFR) $< 1 \, \text{Gyr}^{-1}$ at $z=5.5$ . They classify them into Long Quiescent Galaxies (LQGs) (quenched by $z=6$ ) and Recently Quenched Galaxies (RQGs) (quenched between $z=6$ and $z=5.5$ ).
Dimensionality Reduction & Clustering: To isolate the physical conditions conducive to MQG formation without arbitrary thresholds, the authors applied:
- Principal Component Analysis (PCA): To reduce the dimensionality of 35 halo, galaxy, and environmental variables (including mass, radius, angular momentum, merger history, and overdensities).
- Gaussian Mixture Models (GMM): To probabilistically cluster the PCA space and isolate a distinct subset of haloes hosting MQGs from the general massive galaxy population.
Comparative Analysis: The study compares Thesan results against:
- Thesan-TNG: A run using IllustrisTNG subgrid physics (without explicit radiative transfer) to isolate the impact of the feedback model.
- Other Simulations: Magneticum, Colibre, and Flares to contextualize environmental dependencies.
- Observational Data: JWST-era estimates of MQG number densities.
Metrics: The authors analyzed halo assembly histories using specific accretion gradients ( $\beta$ ), radial skew parameters (to measure anisotropic mass inflow), and overdensities at multiple scales ( $200 \, \text{ckpc}$ , $1 \, \text{cMpc}$ , $3 \, \text{cMpc}$ ).

3. Key Contributions

Identification of a Suppressed Feedback Regime: The paper highlights that a numerical error in Thesan (reduced speed of light in black hole routines) reduced AGN feedback efficiency by a factor of 25 while allowing accretion rates up to 5 $\times$ the Eddington limit. The authors reframe this not as a failure, but as a controlled experiment exploring a physically plausible regime of weakly coupled AGN feedback and super-Eddington growth, which aligns with recent JWST observations of overmassive black holes.
Statistical Framework for MQG Identification: The authors developed a robust, threshold-free statistical method (PCA + GMM) to identify the specific combination of halo properties (mass, potential depth, assembly time, and environment) that predicts early quenching.
Environmental Dependence of Quenching: The study demonstrates that in this suppressed feedback regime, MQGs are strictly associated with overdense environments on megaparsec scales, contrasting with some other simulations (like Magneticum) where quenching can occur in underdense regions due to strong feedback coupling.

4. Key Results

Rapid Assembly via Smooth Accretion: MQGs in Thesan assemble their stellar mass rapidly through smooth halo accretion rather than major mergers. They reside in the most massive, early-forming haloes with deep potential wells.
The Role of the Central Black Hole:
- Due to suppressed feedback, black holes in MQGs grow to unusually high masses ( $M_{BH} \sim 10^8 - 10^9 M_\odot$ ) at early times.
- Quenching occurs only once the black hole reaches a critical mass where kinetic-mode feedback finally becomes effective, despite the weakened coupling.
- This results in a "delayed" but powerful quenching mechanism: prolonged accretion followed by rapid, kinetic-mode suppression of star formation.
Environmental Evolution:
- MQGs reside in significant overdensities ( $\delta$ ) on scales of $200 \, \text{ckpc}$ to $1 \, \text{cMpc}$ .
- LQGs (quenched earlier) inhabit the densest environments, which continue to grow in mass and density even after the galaxy quenches.
- Radial Skew Analysis: The distribution of dark matter around MQG hosts shows increasing positive skew over time, indicating a sustained, anisotropic inflow of matter from neighboring structures. This continuous fuel supply supports the rapid black hole growth required for quenching.
Comparison with Other Models:
- Unlike IllustrisTNG (which produces no MQGs at $z=5.5$ ), Thesan produces a high number density ( $1.033 \times 10^{-5} \, \text{cMpc}^{-3}$ ), matching JWST observational estimates.
- Unlike Magneticum (where quenching can happen in underdense regions), Thesan MQGs are tightly coupled to overdense nodes, suggesting that in weak-feedback regimes, dense environments are a prerequisite for rapid black hole growth and subsequent quenching.
Post-Quenching Growth: The host haloes and their surrounding environments continue to grow in mass and density after the galaxy quenches, suggesting these galaxies will reside in some of the largest structures in the universe by $z=6$ .

5. Significance

Reconciling Theory and Observation: The study provides a physical explanation for the JWST detection of massive, early-quenched galaxies. It suggests that if AGN feedback is weakly coupled or if black holes can grow via super-Eddington accretion in dense, radiation-rich environments, early quenching is a natural outcome.
New Evolutionary Pathway: It identifies a distinct evolutionary channel where smooth accretion and environmental overdensity drive the formation of overmassive black holes, which eventually self-regulate the galaxy through kinetic feedback. This contrasts with merger-driven quenching models.
Guidance for Future Surveys: The statistical framework developed allows researchers to identify candidate MQG hosts in large-scale N-body simulations (which lack baryonic physics) by looking for specific halo and environmental signatures (high mass, early formation, high radial skew, and overdensity). This can guide future deep-field JWST and SKA observations.
Implications for Reionisation Simulations: The findings suggest that excluding AGN feedback in reionisation-era simulations (as done in some zoom-in runs) may lead to inaccurate predictions of massive galaxy populations, as AGN-driven outflows and black hole growth are critical for regulating star formation even at $z > 6$ .

In conclusion, while the Thesan results stem from a numerical artifact, they offer a valuable "limiting case" that illuminates how massive galaxies might evolve under conditions of suppressed feedback, providing a plausible physical mechanism for the early quiescent galaxies now being discovered by JWST.