REVIEW
The star-forming past and quenching of brightest cluster galaxies in TNG-Cluster
Astrophysics of Galaxies · Submitted 5 Oct 2026 · 155 references
Pith's verdict: A solid, honest TNG-Cluster BCG census whose beta≈0.9 energy-budget claim needs a denominator-collapse test before it is pushed as a mechanism.
T0 · AI review
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Referee report
The paper
The paper claims
According to the TNG-Cluster simulation, the brightest cluster galaxies of the most massive haloes quench for the first time not at any characteristic mass but when the cumulative kinetic energy injected by their supermassive black hole…
The reading
What carries the argument
The central object is the dimensionless ratio beta = integral(E_dot_kin,SMBH dt)/E_bind,BCG (Eq. 4), where the numerator integrates the power of all kinetic-mode SMBH energy injections and the denominator is the gas binding energy E_bind = (1/2) sum_i m_i phi_i summed over gas cells within 30 pkpc (Eq. 2). The argument is that a BCG can keep forming stars only while it can still capture gas; once the cumulative kinetic feedback energy overtakes the binding energy, the feedback can unbind and expel the gas, removing the fuel for star formation. This ratio carries the whole argument: the paper shows that the beta near 1 crossing coincides with the first-quenching time and interprets the ~2.3 Gyr gap between the onset of kinetic feedback and quiescence as the time the integral needs to build up to the binding energy—longer for the deep gravitational wells of BCGs than for ordinary massive galaxies.
Extended reading notes
Core claim
According to TNG-Cluster, brightest cluster galaxies do not quench at a characteristic halo, stellar, or supermassive-black-hole mass: the masses at first and last quenching scatter widely and simply ride the median growth track of the population. Instead, a BCG quenches for the first time when the ratio beta = (cumulative kinetic energy injected by its SMBH) divided by (BCG gas binding energy) reaches about one—the median value at first quenching is beta = 0.9 (+2.2/-0.7), the point at which the injected energy is comparable to the binding energy and the gas is expelled. Quenching follows the first kinetic energy injection by a median 2.3 (+1.2/-1.2) Gyr, because the first injections are not enough to unbind the deep potential well of a BCG. The resulting population-level picture is that 93 per cent of BCGs are quenched at z=0 while all were star-forming at z>3; first quenching happens about 9.9 Gyr ago (z~1.7) in a narrow, roughly synchronized window, final quenching about 7.0 Gyr ago with a much wider spread, and the average BCG re-ignites star formation 2.1 times since z=3.
Load-bearing premise
The claim that first quenching happens at beta near 1 leans on first-quenching times interpolated linearly from only fourteen simulation snapshots between z=6.5 and z=0, and on a binding-energy denominator (Eq. 2) that shrinks as the feedback expels the very gas it measures, so the beta crossing is not independent of the removal it is meant to explain.
Editorial extensions
If this is right
- First quenching is roughly synchronized across the population: 80 per cent of BCGs quench for the first time between z=2.2 and z=1.2, about 9.9 Gyr ago.
- The onset of kinetic SMBH feedback is not the moment of quiescence; several billion years of cumulative injection are required, an explicit delay the paper measures as a median 2.3 Gyr.
- Because masses at quenching scatter widely and follow median growth, no single halo, stellar, or SMBH mass marks the quench—quenching times must be predicted by the energy-budget ratio rather than by mass thresholds.
- Rejuvenation is the norm rather than the exception: the average BCG re-ignites about twice since z=3, episodes last roughly 500 Myr, and such activity becomes rarer toward low redshift.
- Even the most massive haloes (M200c above 10^15 Msun) can rejuvenate within the last 5 Gyr, so quiescence at z=0 does not require permanent removal of all cool gas.
Reading between the lines
- The beta-crossing criterion may be partly self-fulfilling: E_bind is computed from the same gas cells the feedback expels, so as gas is removed the denominator shrinks and beta approaches one even before the integral alone would reach the original binding energy; testing with a fixed pre-ejection gas mass would isolate which side of the ratio drives the crossing.
- If the mechanism is genuinely an energy budget rather than a mass threshold, then quenching time should be more tightly ordered by the ratio of integrated kinetic feedback power to binding energy than by any halo, stellar, or black-hole mass—a prediction that could be checked against simulations with different feedback implementations.
- The roughly 2 Gyr delay between first kinetic injection and first quench is a potentially observable signature: spectroscopic campaigns linking the onset of kinetic-mode AGN activity in a BCG to its sSFR decline several gigayears later would test the causal chain beyond simulations.
Original abstract
The Brightest Cluster Galaxies (BCGs) in the most massive galaxy clusters of our Universe are, at present, mostly quiescent, elliptical, and red. However, their formation histories are unclear and likely different from those of massive ellipticals in lower-mass haloes. Simulations face the challenge of simultaneously requiring large volumes to sample massive clusters and sufficient resolution to model their stellar populations. Therefore, we make use of TNG-Cluster, one of the highest-resolution cosmological simulations of galaxy clusters, which returns 352 BCGs at $z=0$ with stellar masses $M_{*, <30 \mathrm{pkpc}} = 10^{11.6-12.9} ~\mathrm{M}_\odot$ and total halo masses $M_{\mathrm{200c}} = 10^{14.3-15.4}~\mathrm{M}_\odot$. 93 per cent of the BCGs are quenched at $z=0$, whereas at redshift $z>3$ all their main progenitors were still star forming. The star-formation histories of individual BCGs are complex, including multiple (re)quenching episodes. They rejuvenate on average $2.1 \pm 1.8$ times since $z=3$. BCGs quench for the first time $9.9^{+0.9}_{-1.4} ~\mathrm{Gyr}$ ago (10th-90th percentiles; $z=1.7^{+0.5}_{-0.5}$), while the last and final quenching event occurred $7.0^{+2.8}_{-5.0} ~\mathrm{Gyr}$ ago ($z=0.8^{+0.8}_{-0.6}$). We find a large scatter in the halo, stellar, and SMBH masses at both first and last quenching, suggesting no characteristic mass at which quenching occurs. Instead, BCGs quench for the first time when the cumulative kinetic energy injected by their SMBH equals the BCG binding energy, corresponding to a delay of $2.3^{+1.2}_{-1.2} ~ \mathrm{Gyr}$ after the first SMBH kinetic energy injection. Namely, TNG-Cluster BCGs quench when they experience significant gas expulsion. Our work shows that BCGs have complex and varied star-formation histories, with multiple rejuvenation episodes, but they ultimately quench due to SMBH feedback.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
The Brightest Cluster Galaxies (BCGs) in the most massive galaxy clusters of our Universe are, at present, mostly quiescent, elliptical, and red. However, their formation histories are unclear and likely different from those of massive ellipticals in lower-mass haloes. Simulations face the challenge of simultaneously requiring large volumes to sample massive clusters and sufficient resolution to model their stellar populations. Therefore, we make use of TNG-Cluster, one of the highest-resolution cosmological simulations of galaxy clusters, which returns 352 BCGs at $z=0$ with stellar masses $M_{*, <30 \mathrm{pkpc}} = 10^{11.6-12.9} ~\mathrm{M}_\odot$ and total halo masses $M_{\mathrm{200c}} = 10^{14.3-15.4}~\mathrm{M}_\odot$. 93 per cent of the BCGs are quenched at $z=0$, whereas at redshift $z>3$ all their main progenitors were still star forming. The star-formation histories of individual BCGs are complex, including multiple (re)quenching episodes. They rejuvenate on average $2.1 \pm 1.8$ times since $z=3$. BCGs quench for the first time $9.9^{+0.9}_{-1.4} ~\mathrm{Gyr}$ ago (10th-90th percentiles; $z=1.7^{+0.5}_{-0.5}$), while the last and final quenching event occurred $7.0^{+2.8}_{-5.0} ~\mathrm{Gyr}$ ago ($z=0.8^{+0.8}_{-0.6}$). We find a large scatter in the halo, stellar, and SMBH masses at both first and last quenching, suggesting no characteristic mass at which quenching occurs. Instead, BCGs quench for the first time when the cumulative kinetic energy injected by their SMBH equals the BCG binding energy, corresponding to a delay of $2.3^{+1.2}_{-1.2} ~ \mathrm{Gyr}$ after the first SMBH kinetic energy injection. Namely, TNG-Cluster BCGs quench when they experience significant gas expulsion. Our work shows that BCGs have complex and varied star-formation histories, with multiple rejuvenation episodes, but they ultimately quench due to SMBH feedback.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | The Brightest Cluster Galaxies (BCGs) in the most massive galaxy clusters of our Universe are, at present, mostly quiescent, elliptical, and red. | background 1.00 | 0.07 | 0.03 | asserted 0.98 |
| s1 | However, their formation histories are unclear and likely different from those of massive ellipticals in lower-mass haloes. | gap 0.96 | 0.06 | 0.02 | hedged possibility 0.98 |
| s2 | Simulations face the challenge of simultaneously requiring large volumes to sample massive clusters and sufficient resolution to model their stellar populations. | background 0.86 | 0.09 | 0.02 | asserted 1.00 |
| s3 | Therefore, we make use of TNG-Cluster, one of the highest-resolution cosmological simulations of galaxy clusters, which returns 352 BCGs at $z=0$ with stellar masses $M_{*, <30 \mathrm{pkpc}} = 10^{11.6-12.9} ~\mathrm{M}_\odot$ and total halo masses $M_{\mathrm{200c}} = 10^{14.3-15.4}~\mathrm{M}_\odot$. 93 per cent of the BCGs are quenched at $z=0$, whereas at redshift $z>3$ all their main progenitors were still star forming. | method 0.62 | 0.84 | 0.14 | asserted 1.00 |
| s4 | The star-formation histories of individual BCGs are complex, including multiple (re)quenching episodes. | result 1.00 | 0.84 | 0.07 | asserted 1.00 |
| s5 | They rejuvenate on average $2.1 \pm 1.8$ times since $z=3$. | result 1.00 | 0.92 | 0.06 | asserted 1.00 |
| s6 | BCGs quench for the first time $9.9^{+0.9}_{-1.4} ~\mathrm{Gyr}$ ago (10th-90th percentiles; $z=1.7^{+0.5}_{-0.5}$), while the last and final quenching event occurred $7.0^{+2.8}_{-5.0} ~\mathrm{Gyr}$ ago ($z=0.8^{+0.8}_{-0.6}$). | result 1.00 | 0.92 | 0.08 | asserted 1.00 |
| s7 | We find a large scatter in the halo, stellar, and SMBH masses at both first and last quenching, suggesting no characteristic mass at which quenching occurs. | result 0.96 | 0.97 | 0.09 | asserted 0.71 |
| s8 | Instead, BCGs quench for the first time when the cumulative kinetic energy injected by their SMBH equals the BCG binding energy, corresponding to a delay of $2.3^{+1.2}_{-1.2} ~ \mathrm{Gyr}$ after the first SMBH kinetic energy injection. | result 0.82 | 0.94 | 0.32 | asserted 1.00 |
| s9 | Namely, TNG-Cluster BCGs quench when they experience significant gas expulsion. | interpretation 0.75 | 0.84 | 0.14 | asserted 1.00 |
| s10 | Our work shows that BCGs have complex and varied star-formation histories, with multiple rejuvenation episodes, but they ultimately quench due to SMBH feedback. | interpretation 0.62 | 0.93 | 0.88 | asserted 1.00 |
What was examined, and what was not
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- P abstract
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Figures
Figures reproduced from arXiv:2610.06634 by the authors.
References
Discussion
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