FLAMINGO: Tracing the co-evolution of hot gas and black holes in galaxy groups and clusters

Emily E. Costello; Ian G. McCarthy; Jaime Salcido; John C. Helly; Joop Schaye; Matthieu Schaller; Robert J. McGibbon

arxiv: 2510.17980 · v2 · pith:5C7FBFKRnew · submitted 2025-10-20 · 🌌 astro-ph.CO · astro-ph.GA

FLAMINGO: Tracing the co-evolution of hot gas and black holes in galaxy groups and clusters

Emily E. Costello , Ian G. McCarthy , Jaime Salcido , John C. Helly , Robert J. McGibbon , Matthieu Schaller , Joop Schaye This is my paper

Pith reviewed 2026-05-21 20:48 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.GA

keywords galaxy clusterssupermassive black holesgas mass fractionhydrodynamical simulationsfeedback processeshalo mass relationcosmic assembly history

0 comments

The pith

Scatter in the gas fraction of galaxy groups and clusters today is set by when their central black holes grew and expelled gas early on.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper uses large-volume hydrodynamical simulations to trace how supermassive black holes and their host haloes evolve together and shape the gas content of groups and clusters. It finds that the correlation between black hole mass and gas fraction reverses with halo mass: negative at lower masses where more massive black holes leave less gas, but positive at higher masses where overmassive black holes sit in gas-richer systems. By following the progenitors of black holes and haloes back in time, the work shows that earlier collapse leads to earlier black hole growth, earlier gas expulsion, and later re-accretion, producing higher present-day gas fractions at fixed halo mass. This establishes that much of the scatter in the gas fraction-halo mass relation is not random but a direct record of early assembly history. The result supplies a concrete prediction that can be checked with future observations of gas and black holes in clusters.

Core claim

For haloes with M500 below 10^13 solar masses, central black hole mass correlates negatively with gas fraction at fixed halo mass, while for haloes between 10^13 and 10^14.5 solar masses the correlation reverses and becomes positive. Tracing the progenitors shows that haloes which collapse earlier form black holes earlier, expel their gas sooner, and then re-accrete it, ending up with higher gas fractions today than later-forming haloes of the same present-day mass. The present-day scatter in the gas fraction-halo mass relation is therefore strongly shaped by the early growth history of black holes and their haloes.

What carries the argument

Progenitor tracing of black holes and haloes through cosmic time, which isolates how the timing of black hole growth regulates the cycle of gas expulsion followed by re-accretion.

If this is right

The scatter in gas fraction at fixed halo mass is not stochastic but is largely inherited from the timing of early black hole feedback.
Haloes that assemble earlier retain more gas today because their expelled material has had more time to re-accrete.
Feedback calibration that uses only the mean gas fraction-halo mass relation misses an important assembly-history dependence.
The reversal in correlation sign marks the mass scale where black hole feedback transitions from permanently removing gas to enabling its later return.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Gas-fraction measurements in clusters could serve as an indirect clock for the epoch of black hole seeding and early growth.
The same early-assembly effect may appear in other baryonic scaling relations such as the stellar-mass to halo-mass relation.
Upcoming wide-field X-ray and Sunyaev-Zel'dovich surveys could test the predicted correlation between gas fraction and halo formation time directly.
If confirmed, the result would tighten the link between black hole feedback models and the baryon budget available for cosmological probes.

Load-bearing premise

That the simulated cycles of gas expulsion and later re-accretion are driven by the actual physics rather than by resolution limits or choices in the subgrid feedback model.

What would settle it

A large observational sample of groups and clusters in which the black-hole-mass versus gas-fraction correlation does not reverse sign near 10^13 solar masses, or in which gas fraction shows no link to halo assembly time.

read the original abstract

The gas mass fraction of galaxy groups and clusters is a key physical quantity for constraining the impact of feedback processes on large-scale structure. While several modern cosmological simulations use the gas fraction-halo mass relation to calibrate their feedback implementations, we note that this relation exhibits substantial intrinsic scatter whose origin has not been fully elucidated. Using the large-volume FLAMINGO hydrodynamical simulations, we examine the role of both central and satellite supermassive black holes (BHs) in shaping this scatter, probing higher halo masses than previously possible. For haloes with M500 < 10^13 Msun, we find that central BH mass correlates strongly and negatively with gas fraction, such that higher BH masses give rise to lower gas fractions at fixed halo mass, consistent with previous studies. Interestingly, however, for 10^13 Msun < M500 < 10^14.5 Msun the correlation reverses and becomes positive, with overmassive BHs residing in haloes with above-average gas fractions. By tracing progenitor BHs and haloes through cosmic time, we show that this behaviour is driven by the expulsion and subsequent re-accretion of halo gas, regulated by the timing of BH growth and feedback. Specifically, haloes that collapse earlier form BHs earlier, leading to earlier gas expulsion and re-accretion and a high gas fraction compared to haloes of the same present-day mass that formed later. Our results demonstrate that present-day scatter in the gas fraction-halo mass relation is strongly shaped by the early growth history of BHs and their haloes, a prediction that can be tested with future observational measurements.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

FLAMINGO finds a sign reversal in central BH mass versus gas fraction at M500 above 10^13 solar masses, traced to early halo collapse via progenitors, but the causal story may be tied to the specific subgrid AGN model.

read the letter

The main thing to know is that in these FLAMINGO simulations, the correlation between central black hole mass and gas fraction in halos flips from negative to positive around 10^13 solar masses, and the authors trace this to earlier halo collapse allowing earlier black hole growth, gas expulsion, and then re-accretion. What the paper does well is use the large volume of FLAMINGO to probe higher mass halos than before and apply progenitor tracking to link present-day properties back to formation history. This provides a concrete mechanism for the scatter in the gas fraction-halo mass relation, which is relevant because many simulations calibrate feedback using that relation. The analysis comes directly from simulation outputs and time-tracing of halos and black holes, avoiding circularity with the target observable. The trends are reported clearly in the abstract, with the reversal and the explanation via early collapse timing. On the soft spots, the interpretation relies heavily on the subgrid AGN feedback model in FLAMINGO. The re-accretion cycle and its timing could be sensitive to how feedback is implemented, such as the heating temperature or efficiency. The paper does not seem to test variations in these parameters or compare against other simulation suites like IllustrisTNG, where gas retention at group scales can differ. This makes it harder to judge if the sign reversal is a robust physical prediction or tied to the specific model choices. Resolution effects or unaccounted environmental factors might also play a role, though the large volume helps with statistics. This paper is for people working on hydrodynamical simulations of galaxy clusters and groups, as well as those using cluster gas fractions for cosmology. Observers planning multi-wavelength studies of black holes and hot gas in clusters might find the testable prediction useful. I would recommend sending this to peer review. The core result is worth evaluating in detail, even if the robustness to feedback variations needs addressing in revisions.

Referee Report

2 major / 2 minor

Summary. The manuscript analyzes the intrinsic scatter in the gas mass fraction-halo mass relation using the large-volume FLAMINGO hydrodynamical simulations. It reports a reversal in the correlation between central black hole mass and gas fraction: negative for M500 < 10^13 Msun (consistent with prior work) but positive for 10^13 < M500 < 10^14.5 Msun. By tracing halo and black hole progenitors through cosmic time, the authors attribute the positive correlation to earlier halo collapse enabling earlier black hole growth, gas expulsion via AGN feedback, and subsequent re-accretion, thereby shaping the present-day scatter.

Significance. If the causal interpretation holds, the work supplies a concrete physical mechanism—rooted in assembly history and feedback timing—for the scatter in a key observable used to calibrate feedback in cosmological simulations. It extends previous analyses to higher halo masses and offers a falsifiable prediction for future observations of gas fractions in groups and clusters.

major comments (2)

[Progenitor tracing and results on correlation reversal] The causal narrative in the abstract and the progenitor-tracing results section attributes the correlation reversal to the expulsion/re-accretion cycle regulated by early BH growth. However, the manuscript does not report tests varying AGN feedback parameters (heating temperature, coupling efficiency, or resolution) within the FLAMINGO framework, nor direct comparisons with other subgrid implementations (e.g., IllustrisTNG). This is load-bearing for the claim that the reported behavior reflects a robust physical process rather than model-specific numerics.
[Discussion of scatter origin] The assertion that early growth history 'strongly shapes' the scatter would benefit from a quantitative decomposition (e.g., fraction of variance in gas fraction at fixed M500 explained by progenitor BH mass or collapse time). Without this, it remains unclear how dominant the mechanism is relative to other sources of scatter such as environment or satellite contributions.

minor comments (2)

[Figures showing correlations] Figure captions and axis labels should explicitly state the halo mass bins and the exact definition of gas fraction (e.g., within R500) to aid reproducibility.
[Introduction] The manuscript would benefit from a short paragraph in the introduction summarizing how the FLAMINGO subgrid AGN model differs from those in previous studies that reported only negative correlations.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and insightful comments, which have helped us identify opportunities to strengthen the manuscript. We respond to each major comment below and indicate the revisions we will implement.

read point-by-point responses

Referee: The causal narrative in the abstract and the progenitor-tracing results section attributes the correlation reversal to the expulsion/re-accretion cycle regulated by early BH growth. However, the manuscript does not report tests varying AGN feedback parameters (heating temperature, coupling efficiency, or resolution) within the FLAMINGO framework, nor direct comparisons with other subgrid implementations (e.g., IllustrisTNG). This is load-bearing for the claim that the reported behavior reflects a robust physical process rather than model-specific numerics.

Authors: We appreciate the referee's concern regarding the robustness of our results. The FLAMINGO simulations employ a specific AGN feedback implementation that was calibrated to match a range of observational constraints. Performing a full parameter variation study or direct comparisons with other codes such as IllustrisTNG would require substantial additional computational resources and is beyond the scope of the current work. In the revised manuscript, we will add a paragraph in the discussion section acknowledging this limitation and emphasizing that our conclusions are based on the fiducial model. We will also note that the physical mechanism identified through progenitor tracing is consistent with expectations from AGN feedback theory. revision: partial
Referee: The assertion that early growth history 'strongly shapes' the scatter would benefit from a quantitative decomposition (e.g., fraction of variance in gas fraction at fixed M500 explained by progenitor BH mass or collapse time). Without this, it remains unclear how dominant the mechanism is relative to other sources of scatter such as environment or satellite contributions.

Authors: We agree that providing a quantitative assessment of how much of the scatter is explained by the early growth history would be beneficial. In the revised version of the manuscript, we will include an additional analysis that decomposes the variance in the gas fraction at fixed halo mass, quantifying the contribution from progenitor black hole mass and halo collapse time. This will allow readers to better evaluate the dominance of this mechanism relative to other potential sources of scatter. revision: yes

Circularity Check

0 steps flagged

Simulation outputs and progenitor tracing yield emergent scatter result without reduction to fitted inputs or self-definitional steps

full rationale

The paper derives its central claim—that present-day scatter in the gas fraction-halo mass relation is shaped by early BH and halo growth history—directly from post-processing of FLAMINGO hydrodynamical simulation snapshots, including explicit progenitor tracing of haloes and central/satellite BHs across cosmic time. No equation or result in the analysis reduces the target scatter or its sign reversal (negative correlation below 10^13 Msun, positive above) to a fitted parameter or redefinition of the gas fraction itself; the simulations are evolved forward with fixed subgrid AGN feedback prescriptions, and the correlations emerge from the time-dependent gas expulsion/re-accretion dynamics. Self-citations to prior FLAMINGO methodology papers describe the simulation volume, resolution, and calibration choices but are not load-bearing for the specific causal narrative, which rests on the internal tracing analysis rather than an imported uniqueness theorem or ansatz. The result is therefore self-contained against external benchmarks and does not exhibit any of the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The analysis depends on the subgrid AGN feedback, cooling, and star-formation prescriptions already implemented and calibrated in the FLAMINGO simulation suite; these prescriptions contain multiple tuned parameters whose values were chosen to reproduce other observables.

axioms (1)

standard math Standard flat Lambda-CDM cosmology with parameters taken from prior observational constraints.
Initial conditions and background cosmology for the FLAMINGO runs.

pith-pipeline@v0.9.0 · 5864 in / 1375 out tokens · 53019 ms · 2026-05-21T20:48:39.827804+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

By tracing progenitor BHs and haloes through cosmic time, we show that this behaviour is driven by the expulsion and subsequent re-accretion of halo gas, regulated by the timing of BH growth and feedback.
IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

haloes that collapse earlier form BHs earlier, leading to earlier gas expulsion and re-accretion

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supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
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