On the unique evolutionary mechanisms of massive quiescent galaxies in the epoch of reionisation
Pith reviewed 2026-05-22 18:55 UTC · model grok-4.3
The pith
Massive quiescent galaxies at z=5.5 form via rapid accretion in dense haloes under suppressed AGN feedback in Thesan.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Thesan contains nine massive quiescent galaxies at z=5.5 within a (95.5 cMpc)^3 volume, unlike IllustrisTNG. Stellar mass builds rapidly through smooth halo accretion in dense regions fed by massive neighbors. Early haloes develop fast-growing potential wells that host massive black holes. A numerical issue suppresses AGN feedback efficiency by a factor of 25 while boosting accretion, permitting extended black hole growth until kinetic-mode quenching activates. Post-quenching, megaparsec overdensities and halo masses continue to grow, so these galaxies will sit in some of the largest structures at later times.
What carries the argument
The suppressed AGN feedback regime, caused by a numerical reduction in feedback efficiency by a factor of 25 and enhanced accretion rates, which permits prolonged black hole growth in early-forming haloes before kinetic quenching.
If this is right
- These galaxies will reside in some of the largest haloes and densest regions of space by z=6.
- Megaparsec-scale overdensities continue growing after the galaxies quench.
- Halo masses keep increasing after quenching.
- The conditions identified isolate the halo and environmental factors most favorable for this evolutionary path.
- Future deep surveys and N-body studies can target analogous systems using these insights.
Where Pith is reading between the lines
- Similar dense environments and early halo formation could drive the high-redshift quiescent population observed by JWST if AGN feedback is comparably weak in nature.
- Comparing the clustering and environments of JWST quiescent galaxies with Thesan predictions would test the role of suppressed feedback.
- N-body simulations of structure formation could incorporate these halo growth histories to model the evolution of massive galaxies at early times.
- The mechanism highlights how local density influences the timing of quenching when global feedback is reduced.
Load-bearing premise
The numerical reduction in AGN feedback efficiency by a factor of 25 produces a representative physical regime for quenching rather than merely an artifact of the simulation code.
What would settle it
Observing the actual number density and environmental properties of massive quiescent galaxies at z approximately 5.5 with JWST or future surveys, and checking whether they match the nine in the simulated volume or show different quenching signatures, would confirm or refute the mechanism.
read the original abstract
We investigate the evolutionary histories of a population of high mass, high redshift, quiescent galaxies in the cosmohydrodynamical simulation Thesan, studying the characteristic properties of their haloes and environments over the epoch of reionisation. Thesan employs a modified version of the Arepo moving-mesh code utilised in IllustrisTNG, which incorporates on-the-fly radiative transfer to couple haloes and galaxies with the evolving radiation field. Thesan exhibits nine massive quiescent galaxies at $z=5.5$, in a $(95.5 \text{cMpc})^3$ volume, with no counterpart in IllustrisTNG. A numerical issue in the simulation reduces AGN feedback efficiency by a factor of 25 while enhancing accretion rates, creating a regime of suppressed feedback. We find their stellar mass assembles rapidly through smooth halo accretion in dense environments, particularly from massive neighbouring structures, while their early-forming haloes develop fast-growing potential wells hosting massive black holes. This suppressed feedback allows prolonged black hole growth before eventual kinetic-mode quenching, providing insight into galaxy evolution under weakened AGN regulation. We find that megaparsec-scale overdensities and halo masses continue growing after quenching, suggesting these galaxies will reside in some of the largest haloes and densest regions of space by $z=6$. With massive quiescent galaxies found in JWST data, the identification of such galaxies in Thesan enables isolation of halo and environmental conditions most conducive to their evolution under this suppressed feedback regime, guiding future deep surveys and N-body simulation studies of analogous systems.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript investigates the evolutionary histories of nine massive quiescent galaxies at z=5.5 identified in the Thesan cosmohydrodynamical simulation (volume (95.5 cMpc)^3), which has no counterparts in IllustrisTNG. It attributes their rapid stellar-mass assembly via smooth halo accretion in dense environments—particularly from massive neighboring structures—to early-forming haloes with fast-growing potential wells hosting massive black holes. A numerical issue in Thesan reduces AGN feedback efficiency by a factor of 25 while enhancing accretion rates, creating a regime of suppressed feedback that permits prolonged black-hole growth before eventual kinetic-mode quenching. The paper concludes that megaparsec-scale overdensities and halo masses continue growing post-quenching, implying these galaxies will occupy some of the largest haloes and densest regions by z=6, with implications for JWST observations of analogous systems.
Significance. If the derived mechanisms remain physically interpretable despite the acknowledged simulation artifact, the work would be significant for highlighting environmental and halo-assembly drivers of quenching in the reionization epoch. The direct comparison to IllustrisTNG and the identification of unique objects in a large volume provide a concrete benchmark for how feedback implementations affect high-redshift quiescent populations. This could usefully guide targeted follow-up in N-body simulations and deep surveys, particularly given JWST detections of early massive quiescent galaxies. The explicit disclosure of the numerical issue is a point of transparency.
major comments (2)
- [Abstract] Abstract: The central claim that the numerical issue 'provides insight into galaxy evolution under weakened AGN regulation' is load-bearing, yet the manuscript provides no quantification of how the factor-of-25 reduction in feedback efficiency (and enhanced accretion) propagates into the reported BH growth timescales, halo potential wells, or post-quenching overdensity evolution. Without such checks or robustness tests, it is unclear whether the evolutionary histories are representative or dominated by the altered feedback/accretion balance.
- [Simulation comparison and results] Comparison to IllustrisTNG and interpretation of uniqueness: The absence of counterparts in IllustrisTNG is presented as evidence for the role of suppressed feedback, but the analysis interprets the simulation's own numerical artifact as the source of physical insight without external observational benchmarks or direct fitting to JWST data. This creates a risk that the claimed environmental drivers and quenching mechanism are circular with the artifact rather than independently validated.
minor comments (2)
- [Abstract] Abstract: The volume is stated as (95.5 cMpc)^3; confirm whether this is the comoving side length and clarify the exact redshift range over which the nine galaxies are selected and tracked.
- [Methods] Notation: The term 'smooth halo accretion' is used to describe stellar-mass assembly; provide a precise definition or reference to how this is quantified (e.g., merger fraction threshold) to avoid ambiguity with minor mergers.
Simulated Author's Rebuttal
We thank the referee for their thoughtful and constructive comments on our manuscript. We address each major comment in detail below and have revised the manuscript accordingly to strengthen the presentation of the numerical artifact and its implications.
read point-by-point responses
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Referee: [Abstract] Abstract: The central claim that the numerical issue 'provides insight into galaxy evolution under weakened AGN regulation' is load-bearing, yet the manuscript provides no quantification of how the factor-of-25 reduction in feedback efficiency (and enhanced accretion) propagates into the reported BH growth timescales, halo potential wells, or post-quenching overdensity evolution. Without such checks or robustness tests, it is unclear whether the evolutionary histories are representative or dominated by the altered feedback/accretion balance.
Authors: We agree that additional quantification would improve clarity. The original manuscript identifies the numerical issue and its qualitative effects but does not provide explicit estimates of its propagation into BH growth timescales or related quantities. In the revised version, we have added a new paragraph in Section 3.2 with analytic estimates showing that the factor-of-25 reduction extends the black-hole growth phase by approximately 50-100 Myr before kinetic-mode quenching sets in, while noting that halo potential well growth and post-quenching overdensity evolution remain dominated by gravitational accretion from the large-scale environment rather than feedback details. We have also included a brief robustness discussion confirming that the reported environmental trends are insensitive to the precise feedback scaling within the explored range. revision: yes
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Referee: [Simulation comparison and results] Comparison to IllustrisTNG and interpretation of uniqueness: The absence of counterparts in IllustrisTNG is presented as evidence for the role of suppressed feedback, but the analysis interprets the simulation's own numerical artifact as the source of physical insight without external observational benchmarks or direct fitting to JWST data. This creates a risk that the claimed environmental drivers and quenching mechanism are circular with the artifact rather than independently validated.
Authors: We appreciate the concern about potential circularity. The absence of counterparts in IllustrisTNG is presented strictly as a demonstration of sensitivity to feedback implementation, not as independent validation. The environmental drivers (rapid smooth accretion in overdensities) and halo assembly histories are extracted directly from the Thesan merger trees and density fields, which are set by the initial conditions and gravity and are only secondarily affected by the AGN artifact. We have not performed direct fitting to JWST data, as this work is a simulation study focused on internal mechanisms rather than observational modeling. In the revised manuscript, we have expanded the final discussion paragraph to explicitly caution that the results apply to the suppressed-feedback regime realized in Thesan and to frame the environmental trends as testable predictions for future N-body and observational work. revision: partial
Circularity Check
No significant circularity; analysis is self-contained simulation interpretation
full rationale
The paper reports simulation outputs from Thesan (with an acknowledged numerical reduction in AGN feedback efficiency) and compares them directly to IllustrisTNG, attributing galaxy properties to the resulting suppressed-feedback regime. No mathematical derivation chain, fitted parameters renamed as predictions, self-definitional equations, or load-bearing self-citations appear in the abstract or described structure. The central claims rest on direct extraction of halo accretion, black-hole growth, and quenching timelines from the simulation snapshots rather than reducing to the inputs by construction. External context from JWST observations is referenced without circular dependence. This is a standard simulation-analysis paper whose results can be checked against the public Thesan data release.
Axiom & Free-Parameter Ledger
free parameters (1)
- AGN feedback efficiency reduction factor =
25
axioms (1)
- domain assumption Standard LCDM cosmology and subgrid physics for star formation, black hole growth, and radiative transfer in Thesan
discussion (0)
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