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arxiv: 2605.16488 · v1 · pith:QLWX4GJ7new · submitted 2026-05-15 · 🌌 astro-ph.CO · astro-ph.GA

Thermostats, Not Engines: A New Picture of Halo Gas Regulation

Hiranya V. Peiris , Andrew Pontzen , Madalina N. Tudorache , Anik Halder , Stephen Thorp , Sinan Deger , Joop Schaye , Matthieu Schaller This is my paper

Pith reviewed 2026-05-20 15:54 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.GA
keywords black hole feedbackentropy ceilinghalo gas regulationbuoyant migrationvirial radiusvirial shocksstar formation rejuvenationgalaxy evolution
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The pith

Black hole feedback regulates gas in massive halos by establishing an entropy ceiling that drives buoyant migration to the virial radius without extra energy input.

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

The paper proposes that black hole feedback functions like a thermostat rather than a continuous engine by imposing an upper limit on the entropy of gas in massive halos. Once this ceiling is set, the gas becomes buoyant and rises to the virial radius using no further energy from the black hole. Supporting simulations show that the entropy of outflows at the virial radius stays independent of halo mass for isotropic thermal feedback while varying with the solid angle for jet feedback. Above a critical halo mass near 10 to the 13.5 to 14 solar masses, virial shocks overpower the ceiling and may restart star formation, matching recent low-redshift data on star formation rates and galaxy morphologies.

Core claim

Black hole feedback regulates gas in massive halos by establishing an entropy ceiling; the resulting buoyant gas migrates to the virial radius with no additional energy input required. Supporting simulations back this picture: at the virial radius, outflow entropy is mass-independent for isotropic thermal feedback but depends on the solid angle of directly heated gas for jet feedback. Above a critical halo mass, virial shocks overwhelm the ceiling, predicting rejuvenation of star formation in the most massive galaxies, supported by new low-redshift evidence from star formation rates and morphologies.

What carries the argument

The entropy ceiling established by black hole feedback, which imposes an upper limit on gas entropy and thereby enables passive buoyant migration outward.

If this is right

  • Gas regulation in massive halos proceeds through passive buoyant transport once the entropy ceiling is set.
  • Outflow entropy at the virial radius remains independent of halo mass under isotropic thermal feedback.
  • Jet feedback produces virial-radius entropy that scales with the solid angle of the directly heated gas.
  • Virial shocks dominate above a critical mass near 10 to the 13.5 to 14 solar masses and restart star formation.
  • The mechanism accounts for observed low-redshift star formation rates and galaxy morphologies.

Where Pith is reading between the lines

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

  • The thermostat picture may offer a single regulating principle that connects feedback behavior across a wider range of halo masses.
  • Targeted observations of circumgalactic entropy profiles could test whether the predicted mass independence holds in real systems.
  • Analogous buoyancy ceilings might operate in other feedback-regulated astrophysical environments.
  • Mapping how the critical mass threshold changes with redshift would extend the model across cosmic time.

Load-bearing premise

That the entropy ceiling set by black hole feedback allows the gas to migrate buoyantly to the virial radius with no further energy input required.

What would settle it

A direct measurement of outflow entropy at the virial radius that shows clear dependence on halo mass even under isotropic thermal feedback, or the lack of rejuvenated star formation in galaxies hosted by halos above 10 to the 14 solar masses.

Figures

Figures reproduced from arXiv: 2605.16488 by Andrew Pontzen, Anik Halder, Hiranya V. Peiris, Joop Schaye, Madalina N. Tudorache, Matthieu Schaller, Sinan Deger, Stephen Thorp.

Figure 1
Figure 1. Figure 1: FIG. 1. Entropy as a function of halo mass at [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Specific star formation rate as a function of stellar mass, [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Entropy as a function of halo mass at 2 [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
read the original abstract

We propose that black hole feedback regulates gas in massive halos by establishing an entropy ceiling; the resulting buoyant gas migrates to the virial radius with no additional energy input required. The FLAMINGO simulations support this picture: at the virial radius, outflow entropy is mass-independent for isotropic thermal feedback but depends on the solid angle of directly heated gas for jet feedback. Above a critical halo mass $M_\rm{crit} \approx 10^{13.5\text{--}14}\, M_\odot$, virial shocks overwhelm the ceiling, predicting rejuvenation of star formation in the most massive galaxies, supported by new low-redshift evidence from star formation rates and morphologies.

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.

Referee Report

2 major / 1 minor

Summary. The manuscript proposes that black hole feedback regulates gas in massive halos by establishing an entropy ceiling, allowing buoyant gas to migrate to the virial radius with no additional energy input required. FLAMINGO simulations are cited to show mass-independent outflow entropy at r_vir for isotropic thermal feedback (but solid-angle dependent for jets). Above M_crit ≈ 10^{13.5–14} M_⊙, virial shocks overwhelm the ceiling, predicting star-formation rejuvenation in the most massive galaxies, consistent with low-redshift SFR and morphology data.

Significance. If the central claims hold, the work supplies a conceptually distinct thermostat framework for halo gas regulation that could unify simulation results on feedback modes and make falsifiable predictions for galaxy rejuvenation. The explicit use of FLAMINGO to demonstrate mass-independent entropy at the virial radius for thermal feedback constitutes a concrete, simulation-grounded illustration that strengthens the interpretive shift from engine-like to boundary-condition models.

major comments (2)
  1. [Abstract] Abstract: The assertion that buoyant migration to r_vir occurs with literally 'no additional energy input required' rests on an untested assumption of entropy preservation during transit. The reported mass-independent outflow entropy at r_vir is an endpoint measurement; it does not demonstrate that entrainment, mixing, or radiative losses do not alter entropy en route, which is load-bearing for the thermostat picture.
  2. [FLAMINGO simulation results] FLAMINGO results and M_crit identification: The critical mass threshold and the entropy-independence claim are extracted from the same simulation suite used to illustrate the model. Without explicit data-selection criteria, quantitative fits, or robustness tests against parameter variations, it is unclear whether the reported behaviors are robust or partly self-referential.
minor comments (1)
  1. [Notation] The notation M_⊙ and the approximate range for M_crit should be defined on first use in the main text rather than only in the abstract.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed comments on our manuscript. We address each major comment point by point below, providing our response and indicating where revisions will be made to strengthen the presentation.

read point-by-point responses

  1. Referee: [Abstract] The assertion that buoyant migration to r_vir occurs with literally 'no additional energy input required' rests on an untested assumption of entropy preservation during transit. The reported mass-independent outflow entropy at r_vir is an endpoint measurement; it does not demonstrate that entrainment, mixing, or radiative losses do not alter entropy en route, which is load-bearing for the thermostat picture.

    Authors: We appreciate the referee's emphasis on this distinction. The FLAMINGO simulations are full-physics hydrodynamical runs that self-consistently evolve entrainment, mixing, radiative cooling, and all other relevant processes. The reported mass-independent entropy at r_vir for isotropic thermal feedback is therefore not merely an endpoint measurement but the net outcome after all transit effects have acted. This supports our claim that the entropy ceiling set by black hole feedback enables buoyant migration to r_vir without requiring additional energy input from the feedback mechanism itself. To address the concern, we have revised the abstract and added a clarifying paragraph in the methods and results sections explicitly noting that the simulations already incorporate these processes and that the mass-independence persists despite them. revision: yes

  2. Referee: [FLAMINGO simulation results] The critical mass threshold and the entropy-independence claim are extracted from the same simulation suite used to illustrate the model. Without explicit data-selection criteria, quantitative fits, or robustness tests against parameter variations, it is unclear whether the reported behaviors are robust or partly self-referential.

    Authors: We agree that additional methodological transparency is warranted. In the revised manuscript we will expand the relevant section to provide explicit criteria for identifying outflowing gas and measuring entropy at r_vir, include quantitative fits to the mass-independence relation, and report robustness checks against available variations in feedback parameters within the FLAMINGO suite. These additions will clarify that the results are not self-referential but emerge from the simulation outputs under well-defined analysis choices. revision: yes

Circularity Check

0 steps flagged

No significant circularity; model is interpretive framework supported by independent simulation outputs and external observations

full rationale

The paper proposes an interpretive picture in which black hole feedback establishes an entropy ceiling, enabling buoyant migration to the virial radius without further energy input. This picture is illustrated and supported by FLAMINGO simulation measurements of mass-independent outflow entropy at r_vir for thermal feedback, with a critical halo mass identified from the same runs where virial shocks dominate. The subsequent prediction of rejuvenated star formation above M_crit is explicitly tied to separate low-redshift observational evidence on star-formation rates and morphologies rather than to any re-use of the simulation data. No equations, fitted parameters, or self-citations are shown to reduce the central claims to tautological redefinitions or to the same inputs by construction. The derivation therefore remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 1 invented entities

The proposal introduces the entropy ceiling as a new regulatory concept and relies on simulation-derived thresholds whose independence from fitting choices is not demonstrated in the abstract.

free parameters (1)
  • M_crit = 10^{13.5-14} M_sun
    Approximate critical halo mass separating regimes where the entropy ceiling is or is not overwhelmed by virial shocks.
axioms (1)
  • domain assumption Black hole feedback can impose a well-defined entropy ceiling on halo gas that remains stable enough for buoyancy to dominate transport.
    This is the load-bearing premise that converts feedback into a passive regulatory limit rather than an active energy source.
invented entities (1)
  • entropy ceiling no independent evidence
    purpose: To act as a thermostat that limits gas cooling and enables buoyant outflow without continuous energy injection.
    New conceptual entity introduced to reframe black hole feedback; no independent falsifiable prediction outside the simulations is stated in the abstract.

pith-pipeline@v0.9.0 · 5676 in / 1508 out tokens · 64587 ms · 2026-05-20T15:54:41.258037+00:00 · methodology

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Reference graph

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