Recognition: 2 theorem links
· Lean TheoremFirst results from the IllustrisTNG simulations: the galaxy color bimodality
Pith reviewed 2026-05-10 20:26 UTC · model grok-4.3
The pith
Supermassive black hole feedback in its low-accretion state drives the blue-to-red color transition of galaxies.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In the TNG model the primary driver of galaxy color transition is supermassive blackhole feedback in its low-accretion state. Across the entire population the median color transition timescale is about 1.6 Gyr and drops for increasingly massive galaxies. Redder galaxies at fixed stellar mass have lower star formation rates, gas fractions, and gas metallicities, with older stellar populations and weaker large-scale interstellar magnetic fields.
What carries the argument
Supermassive black hole feedback operating in its low-accretion state, which quenches star formation and moves galaxies onto the red sequence.
If this is right
- Galaxies with stellar mass above 10^11 solar masses that redden at redshift below 1 add on average 25 percent of their final mass after becoming red.
- About 18 percent of such massive galaxies acquire half or more of their final stellar mass while already on the red sequence.
- At fixed stellar mass, redder galaxies show lower gas content, lower star formation rates, older stars, and weaker magnetic fields than bluer ones.
Where Pith is reading between the lines
- Direct comparison of the simulated 1.6 Gyr transition time to observed green-valley crossing times in surveys would test whether the feedback prescription matches reality.
- The model implies that AGN activity levels should correlate with the fraction of galaxies caught mid-transition in different environments.
- Extending the same simulation suite to higher redshifts would show how the color bimodality assembles over cosmic time.
Load-bearing premise
The sub-grid prescriptions for supermassive black hole feedback and other baryonic processes in the TNG model accurately capture the physical mechanisms that quench star formation in real galaxies.
What would settle it
An observational measurement of the median time galaxies take to cross from blue to red colors that differs substantially from 1.6 Gyr.
read the original abstract
We introduce the first two simulations of the IllustrisTNG project, a next generation of cosmological magnetohydrodynamical simulations, focusing on the optical colors of galaxies. We explore TNG100, a rerun of the original Illustris box, and TNG300, which includes 2x2500^3 resolution elements in a volume twenty times larger. Here we present first results on the galaxy color bimodality at low redshift. Accounting for the attenuation of stellar light by dust, we compare the simulated (g-r) colors of 10^9 < M*/Msun < 10^12.5 galaxies to the observed distribution from the Sloan Digital Sky Survey (SDSS). We find a striking improvement with respect to the original Illustris simulation, as well as excellent quantitative agreement in comparison to the observations, with a sharp transition in median color from blue to red at a characteristic M* ~ 10^10.5 Msun. Investigating the build-up of the color-mass plane and the formation of the red sequence, we demonstrate that the primary driver of galaxy color transition in the TNG model is supermassive blackhole feedback in its low-accretion state. Across the entire population we measure a median color transition timescale dt_green of ~1.6 Gyr, a value which drops for increasingly massive galaxies. We find signatures of the physical process of quenching: at fixed stellar mass, redder galaxies have lower SFRs, gas fractions, and gas metallicities; their stellar populations are also older and their large-scale interstellar magnetic fields weaker than in bluer galaxies. Finally, we measure the amount of stellar mass growth on the red sequence. Galaxies with M* > 10^11 Msun which redden at z<1 accumulate on average ~25% of their final z=0 mass post-reddening; at the same time, ~18% of such massive galaxies acquire half or more of their final stellar mass while on the red sequence.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces the first results from the IllustrisTNG project, presenting TNG100 (a rerun of the original Illustris volume) and TNG300 (a larger 300 Mpc box). After applying dust attenuation, the simulated (g-r) colors of galaxies with 10^9 < M*/Msun < 10^12.5 show striking improvement over the original Illustris run and excellent quantitative agreement with the SDSS color-mass distribution, including a sharp blue-to-red transition at M* ~ 10^10.5 Msun. The authors identify supermassive black hole feedback in its low-accretion (kinetic wind) state as the primary driver of the color transition, measure a median transition timescale dt_green of ~1.6 Gyr (decreasing with mass), and report that at fixed stellar mass redder galaxies exhibit lower SFRs, gas fractions, older stellar populations, and weaker magnetic fields. They also quantify post-quenching stellar mass growth on the red sequence for M* > 10^11 Msun galaxies.
Significance. If the results hold, the work is significant for demonstrating that an updated cosmological simulation suite can reproduce the observed galaxy color bimodality with high fidelity across a wide mass range and large volume. The reported median transition timescale and the link to low-accretion BH feedback offer concrete, falsifiable predictions that can be tested against observations of green-valley galaxies and quenching timescales. The improvement relative to Illustris, achieved through a combination of model updates, advances the field by showing how sub-grid baryonic physics can be calibrated to match multi-wavelength galaxy properties without obvious fine-tuning.
major comments (2)
- [build-up of the color-mass plane / red sequence formation] Section discussing the build-up of the color-mass plane and formation of the red sequence: the assertion that low-accretion-state BH feedback is the primary driver rests on correlations (redder galaxies at fixed mass have lower SFR, gas fraction, older stars, weaker B-fields) and improvement over Illustris, but the manuscript does not present a control simulation that disables only the kinetic wind mode while retaining all other TNG updates (revised stellar feedback, magnetic fields, BH seeding/accretion). Without such isolation or explicit timing analysis showing that kinetic energy injection systematically precedes the SFR drop and color change in individual galaxy histories, the causal primacy over other baryonic processes remains an inference rather than a demonstrated necessity.
- [color transition timescale measurement] Abstract and results on dt_green: the median color transition timescale of ~1.6 Gyr is reported across the population, yet the manuscript does not detail how this timescale is operationally defined (e.g., time spent in a specific color window, or time from last star-formation episode to red-sequence entry) or show robustness checks against variations in the color cut or dust model; this definition is load-bearing for the claim that the transition is rapid and mass-dependent.
minor comments (2)
- [methods / figures] Figure captions and methods section: the dust attenuation model (including how it is applied to stellar particles and its dependence on gas metallicity or geometry) should be described more explicitly so that the quantitative agreement with SDSS can be reproduced by others.
- [throughout] Notation: the symbol dt_green is introduced without a prior definition in the text; a brief equation or parenthetical definition at first use would improve clarity.
Simulated Author's Rebuttal
We thank the referee for their constructive and detailed report, which highlights important points for clarification and strengthening of the manuscript. We address each major comment below and have made revisions to improve the presentation of our results.
read point-by-point responses
-
Referee: Section discussing the build-up of the color-mass plane and formation of the red sequence: the assertion that low-accretion-state BH feedback is the primary driver rests on correlations (redder galaxies at fixed mass have lower SFR, gas fraction, older stars, weaker B-fields) and improvement over Illustris, but the manuscript does not present a control simulation that disables only the kinetic wind mode while retaining all other TNG updates (revised stellar feedback, magnetic fields, BH seeding/accretion). Without such isolation or explicit timing analysis showing that kinetic energy injection systematically precedes the SFR drop and color change in individual galaxy histories, the causal primacy over other baryonic processes remains an inference rather than a demonstrated necessity.
Authors: We agree that a dedicated control simulation isolating only the kinetic wind mode would provide more direct evidence of causality. Running such a simulation is not feasible within the scope of this work due to the substantial computational cost. The TNG model incorporates the low-accretion kinetic mode specifically to address the color bimodality shortcomings of the original Illustris run, and the reported correlations are tied to the implementation of this mode. In the revised manuscript we will add an explicit timing analysis of individual galaxy histories, showing the sequence of BH accretion state changes relative to SFR decline and color evolution. We have also revised the text to describe the low-accretion BH feedback as the primary driver 'within the TNG model' rather than claiming a model-independent demonstration. revision: partial
-
Referee: Abstract and results on dt_green: the median color transition timescale of ~1.6 Gyr is reported across the population, yet the manuscript does not detail how this timescale is operationally defined (e.g., time spent in a specific color window, or time from last star-formation episode to red-sequence entry) or show robustness checks against variations in the color cut or dust model; this definition is load-bearing for the claim that the transition is rapid and mass-dependent.
Authors: We thank the referee for noting the need for a precise definition of dt_green. This timescale is defined as the time for a galaxy to cross from the blue cloud into the red sequence, specifically the interval between (g-r) = 0.55 and (g-r) = 0.75 while tracking the galaxy's main progenitor branch. We will expand the methods section with this operational definition, including the exact color thresholds and how galaxies are selected. We will also add robustness tests varying the color boundaries by ±0.05 mag and using an alternative dust model; these confirm that the median value of ~1.6 Gyr and its mass trend are stable. The revised manuscript includes these details and an accompanying figure. revision: yes
Circularity Check
No significant circularity in simulation-based attribution of galaxy color transitions
full rationale
The paper runs the TNG cosmological simulations with fixed sub-grid prescriptions (including low-accretion BH feedback) and then extracts statistical properties from the outputs, such as the color-mass relation, correlations at fixed mass, and a median dt_green transition timescale of ~1.6 Gyr measured directly from galaxy star-formation histories. These quantities are compared to independent SDSS observations for validation. The attribution of the blue-to-red transition primarily to BH feedback is an inference drawn from the included physics and observed correlations in the simulated population, not a quantity fitted to the color data or defined in terms of itself. No equations or steps reduce the reported results to the inputs by construction. Self-citations to prior Illustris work supply simulation methodology context but are not load-bearing for the new claims. The derivation chain remains self-contained against external observational benchmarks.
Axiom & Free-Parameter Ledger
free parameters (1)
- AGN and stellar feedback parameters
axioms (1)
- standard math Standard Lambda-CDM cosmological model
Lean theorems connected to this paper
-
Cost.FunctionalEquationwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
the primary driver of galaxy color transition in the TNG model is supermassive blackhole feedback in its low-accretion state. Across the entire population we measure a median color transition timescale dt_green of ~1.6 Gyr
-
Foundation.DAlembert.Inevitabilitybilinear_family_forced unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
TNG incorporates multiple simultaneous changes relative to original Illustris (updated stellar feedback, magnetic fields, revised BH seeding and accretion)
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- 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.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Forward citations
Cited by 24 Pith papers
-
Origins of Extreme Emission-Line Ratios in z > 3 Galaxies: Insights from the Lumen Model
Lumen modeling of IllustrisTNG50 shows that high ionization parameters from massive star clusters plus enhanced nitrogen abundances are needed to reproduce the extreme [OIII]/Hβ, [OIII]/[OII], and [NII]/Hα ratios seen...
-
The Thermodynamic and Kinematic Evolution of Circumgalactic Gas around $z=1$ in the IllustrisTNG model
In the IllustrisTNG model, CGM gas around z=1 galaxies mixes quickly and separates into cold inner and warm-hot outer phases within 500 Myr due to feedback, with kinematic decorrelation over 400 Myr and ion-specific p...
-
Universal Dark-matter Density Profiles of Cosmic Filaments
Filament dark matter density profiles become nearly universal when scaled by terminal halo virial radii, with the inner cusp produced by embedded halos and the smooth component showing a flat core that evolves from sm...
-
Probing the Hot Gaseous Halos of Milky Way-like Galaxies in the TNG50 simulation
TNG50 MW analogues reproduce global soft X-ray luminosity, inner surface brightness, emission measure and O VII absorption but show too-steep radial decline in X-ray brightness and 65% lower O VIII absorption than obs...
-
Cosmological constraints from the small scale clustering of Emission Line Galaxies
SHAMe-SF modeling of small-scale DESI ELG clustering delivers 6% precision on σ8 and Ωm h², matching full DR1 results with 1% volume.
-
Classifying Supermassive Black Hole Growth Regimes to Observables Across Cosmological Simulations with Forecasts for LSST
Machine learning on cosmological simulations achieves 91-94% accuracy classifying over-massive versus under-massive SMBH growth regimes from LSST photometry, with 83-89% cross-simulation transfer accuracy driven prima...
-
Inferring the star-formation histories of massive quiescent galaxies with BAGPIPES: Evidence for multiple quenching mechanisms
BAGPIPES fitting of 9289 massive quiescent galaxies shows most SFHs rise gradually then quench in 1-2 Gyr, with faster quenching at z>1 and slower at z<1, interpreted as multiple AGN feedback and gas-supply mechanisms.
-
Utilizing Dispersion Measure of Fast Radio Bursts to Probe the Intergalactic Medium Turbulence
Fast radio burst dispersion measures exhibit scaling consistent with two-dimensional Kolmogorov turbulence in the intergalactic medium, constraining the outer scale to several megaparsecs.
-
Environmental Quenching of High-Redshift Galaxies: Interpreting JWST Observations with Simulations
Multiple galaxy formation simulations show that low-mass quenched galaxies at z>3 are predominantly environmentally quenched satellites, often only temporarily so, and match JWST observations.
-
The LISA Astrophysics MBHcatalogues Project: A comparison of predictions of simulated massive black hole binaries
A large collaboration compiles and compares merger rate predictions for massive black holes across multiple galaxy formation models to forecast LISA detections and quantify uncertainties.
-
Dynamically cold discs in high-redshift galaxies: comparison between ALMA observations and TNG50
TNG50 shows most massive high-z star-forming galaxies are dynamically hotter than ALMA data indicate, with rare cold discs forming from aligned accretion and evolving into one-third discs and two-thirds early-type gal...
-
Analysis of synthetic OVI absorption associated with galaxy groups in SIMBA and TNG50 simulations
Both SIMBA and TNG50 simulations underpredict OVI covering fractions around galaxy groups compared to observations, with most absorbers being gravitationally bound.
-
Galaxy Populations in the IllustrisTNG Caustic Skeleton
Galaxy properties in IllustrisTNG form a continuum across the multiscale caustic skeleton, with formation time of web components influencing colors and star formation activity.
-
Galaxy discs regulate the growth of supermassive black holes
Disc galaxies inhibit supermassive black hole growth by preserving rotational support in central gas, while mergers in ellipticals disrupt this support and enable rapid accretion.
-
Tracing the relic nature of compact galaxies through their globular cluster systems
Globular cluster mass fractions, in-situ fractions, metallicity spreads, and spatial profiles in simulated compact galaxies can identify massive relic analogs with early assembly histories.
-
The Galaxy Luminosity Functions in ASTRID: Predictions for LSST
ASTRID simulation with dust model calibrated to SDSS at z=0 produces validated luminosity functions and LSST-ready mock catalogs of 378 million galaxies with predicted number counts in ugrizy bands from z=0 to 2.
-
First Light And Reionization Epoch Simulations (FLARES) XXI: The UV Indices of Galaxies in the Early Universe
Simulations of high-redshift galaxies show the 1719 Å UV index reliably traces stellar metallicity while others are more sensitive to star formation history.
-
Identification of Compact Groups of Galaxies in IllustrisTNG300
Compact galaxy groups identified in TNG300 show that ~80% of velocity-selected groups are line-of-sight interlopers, with a stellar-mass versus velocity-dispersion scaling relation serving as an effective diagnostic.
-
A statistical look on kinematic planes of satellite galaxies II: The physics behind their early formation in TNG50 MW/M31-like galaxies
Early kinematically persistent planes of satellite galaxies are fossil remnants of high-redshift anisotropic mass collapse along the principal directions of the local cosmic web during the fast assembly phase of host halos.
-
Probing the Hot Gaseous Halos of Milky Way-like Galaxies in the TNG50 simulation
TNG50 halos match observed inner X-ray luminosity and O VII absorption but show too-steep surface brightness decline at large radii and underproduce O VIII absorption, indicating overly compact structures and missing ...
-
Revisiting the distance and the globular cluster system of the remarkable galaxy UDG1 in the NGC 5846 group
New SBF distance confirms NGC5846_UDG1 lies in the NGC 5846 group and reconciles prior GC counts to ~50 members, supporting a massive dark matter halo.
-
Low-ionization Metal Absorption at $0.7 \lesssim z \lesssim 2$ Confronting Cosmological Simulations with Observations
IllustrisTNG with a uniform UVB reproduces observed MgI, MgII and FeII column-density PDFs and low-EW MgII incidence but underestimates the number of strong MgII systems and fails to capture their rise toward z~2.
-
Dynamical evolution of Milky Way globular clusters on the cosmological timescale II. Terzan 2, 4, and 5 mass loss and collision tracking
N-body simulations show that mutual interactions between Terzan 2, 4, and 5 raise mass-loss rates for the smaller clusters and drive prolate deformations absent in isolated runs.
-
Massive black holes and their galaxies
A review summarizing detection methods, population statistics, and coevolution of supermassive black holes with host galaxies from early universe observations and simulations.
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.