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arxiv: 2512.02127 · v3 · submitted 2025-12-01 · 🌌 astro-ph.GA

Ripples in the baryon to dark matter ratio in ΛCDM: implications for galaxy formation

Owen Jessop , Adrian Jenkins , Andrew Pontzen , Joop Schaye , Matthieu Schaller , John C. Helly This is my paper

Pith reviewed 2026-05-17 02:33 UTC · model grok-4.3

classification 🌌 astro-ph.GA
keywords isocurvature perturbationsbaryon fractiongalaxy formationhigh-redshift galaxieshydrodynamical simulationsLambda CDMstar formation ratesdark matter
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The pith

Small natural variations in the baryon-to-dark-matter ratio cause early galaxies to form with 5 percent fewer baryons and 12 percent less star formation at redshift 8.

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

In the standard cosmological model, tiny local imbalances between baryons and dark matter are expected to exist on the scales where galaxies assemble. These imbalances are locked in early and cause collapsing halos to start with less than the average share of baryons, with the strongest effect in the first halos to collapse. Hydrodynamical simulations that include these variations show a clear drop in both the average baryon content and the rate at which stars form inside resolved halos at redshift 8. The suppression is almost the same across halo masses and fades rapidly toward the present day. Because upcoming telescopes will routinely observe galaxies at these early times, accounting for the variations will be needed to make reliable predictions about how the first galaxies and black holes grew.

Core claim

Baryon-CDM isocurvature perturbations represent local, compensated variations in the ratio of baryon to dark-matter density that freeze in with an RMS amplitude of 1.5 percent on the Lagrangian scale of a 10^11 solar-mass halo. These perturbations are anti-correlated with the matter overdensity field, so halos form with baryon fractions below the cosmic mean and earlier-collapsing halos experience stronger suppression. Three otherwise identical FLAMINGO hydrodynamical simulations demonstrate that, at redshift 8, the perturbations reduce the mean baryon fraction of resolved halos by 5 percent and their star-formation rates by 12 percent relative to a simulation without the perturbations; the

What carries the argument

Baryon-CDM isocurvature perturbations imposed on the initial conditions of hydrodynamical simulations, which are anti-correlated with the matter overdensity and produce systematically lower baryon fractions in collapsing halos.

If this is right

  • Earlier-collapsing halos exhibit stronger baryonic suppression than later ones.
  • The suppression of baryon fraction and star formation falls to 0.1 percent and 1 percent by redshift zero.
  • A simple spherical-collapse model reproduces the average baryon-fraction reduction seen in the simulations.
  • Predictions for early galaxy and black-hole growth must incorporate these perturbations to remain robust against JWST-era observations.

Where Pith is reading between the lines

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

  • The mass-independent character of the effect suggests it could systematically shift the high-redshift galaxy stellar-mass function relative to standard simulations.
  • Because the perturbations are strongest at the highest redshifts, they may alter the timing of the first star-forming episodes more than the final z=0 properties.
  • If confirmed, the effect provides a new consistency test between large-scale structure measurements and the observed properties of the earliest galaxies.

Load-bearing premise

The FLAMINGO galaxy formation model together with the chosen implementation of isocurvature modes accurately captures how galaxies respond to these small initial density variations without large numerical artifacts or missing physics.

What would settle it

A measurement of the average baryon fraction inside dark-matter halos at redshift 8 that is statistically inconsistent with the 5 percent suppression predicted when isocurvature modes are included.

Figures

Figures reproduced from arXiv: 2512.02127 by Adrian Jenkins, Andrew Pontzen, John C. Helly, Joop Schaye, Matthieu Schaller, Owen Jessop.

Figure 1
Figure 1. Figure 1: 𝑧 = 0 linear density fields computed with CLASS for the matter overdensity field, 𝛿m, the isocurvature overdensity field, 𝛿bc, and the orthogonal isocurvature overdensity field, 𝛿 ⊥ bc, see Eqs. (1) to (3). The fields have been smoothed with a spherical top-hat filter on the Lagrangian scale corresponding to a 1011 M⊙ halo (𝑅 = 0.85 Mpc). The cosmological parameters used correspond to the fiducial cosmolog… view at source ↗
Figure 2
Figure 2. Figure 2: Top panel: The time evolution of the comoving baryon mass den￾sity in resolved halos, 𝜌 halos b (𝑧), for our three different simulations. Bottom panel: The time evolution of the baryonic mass density in the FID and INV simulations relative to the NULL simulation, expressed as a ratio. The dotted green lines in the bottom panel represent the predictions of the theoretical model derived in Section 2.2, Eq. (… view at source ↗
Figure 3
Figure 3. Figure 3: The time evolution of the ratio of various global quantities for the FID/NULL simulations (thick blue lines) and the INV/NULL simulations (thin red lines). The dotted green lines displayed in panel (a) represent the prediction of the theoretical model derived in Section 2.2, Eq. (9). Jackknife errors are computed for each simulation, propagated through to the ratios, and shown as shaded regions. Panel (a):… view at source ↗
read the original abstract

We use the FLAMINGO galaxy formation model to quantify the impact of baryon-CDM isocurvature perturbations on galaxy formation in $\Lambda$CDM. In linear theory, these perturbations represent local, compensated variations in the ratio between the baryon and CDM densities; they freeze in amplitude at late times, with an RMS amplitude of $1.5\%$ on the Lagrangian scale of a $10^{11}\,\rm M_\odot$ halo ($0.85\, \rm{Mpc}$). Although such perturbations arise naturally within $\Lambda$CDM, most cosmological simulations and semi-analytic models to date omit them. These perturbations are strongly anti-correlated with the matter overdensity field such that halos form with baryon fractions below the cosmic mean, with earlier-collapsing halos exhibiting stronger baryonic suppression. To isolate the galaxy response, we analyse three hydrodynamical simulations with identical initial matter overdensity fields that: i) include isocurvature modes, ii) omit them, or iii) invert their amplitude. At $z=8$, isocurvature perturbations reduce the mean baryon fraction and star formation rates of resolved halos by $ 5\%$ and $ 12\%$, respectively, relative to the null-isocurvature case. These effects are almost independent of halo mass and diminish steadily with time, reaching $ 0.1\%$ and $ 1\%$ by $z=0$. We develop a model based on spherical collapse that accurately reproduces the mean baryon fraction suppression. As high-redshift observations become increasingly routine, incorporating isocurvature perturbations into simulations and semi-analytic models will be important for robust predictions of early galaxy and black hole formation in the JWST era.

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

0 major / 2 minor

Summary. The paper quantifies the impact of baryon-CDM isocurvature perturbations on galaxy formation in ΛCDM using the FLAMINGO hydrodynamical model. Three simulations share identical initial matter overdensity fields but differ in their treatment of isocurvature modes (included, omitted, or inverted). At z=8 the perturbations suppress the mean baryon fraction of resolved halos by 5% and star formation rates by 12% relative to the null-isocurvature run; both effects are nearly mass-independent and decay to 0.1% and 1% by z=0. A spherical-collapse model is shown to reproduce the mean baryon-fraction shift.

Significance. If the quantitative results hold, the work demonstrates that a naturally occurring but frequently omitted feature of ΛCDM initial conditions can produce percent-level shifts in high-redshift baryon fractions and SFRs. The controlled comparison of three dedicated simulations with fixed matter overdensity fields, together with the independent spherical-collapse reproduction of the mean effect, supplies direct numerical support for the central claims and strengthens the case for including isocurvature modes in future high-z modeling.

minor comments (2)
  1. The methods section should explicitly state the halo mass resolution limit and the minimum number of particles required for a halo to be considered 'resolved' when reporting the 5% and 12% mean suppressions at z=8.
  2. Figure 3 (or equivalent) showing the time evolution of the baryon-fraction offset would benefit from shaded uncertainty bands derived from the halo-to-halo scatter or bootstrap resampling to allow readers to judge the statistical significance of the reported decay toward z=0.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of our work and for recommending minor revision. The report correctly summarizes the key results and highlights the controlled nature of the simulation comparison. Since no specific major comments were raised, we address the overall evaluation below.

Circularity Check

0 steps flagged

No significant circularity in simulation-based derivation chain

full rationale

The paper's primary results derive from direct outputs of three controlled hydrodynamical simulations (identical initial matter overdensity fields, toggling isocurvature inclusion/omission/inversion) using the FLAMINGO model. Reported 5% baryon-fraction and 12% SFR reductions at z=8 are measured quantities from resolved halos in these runs, not fitted parameters or self-defined quantities. The spherical-collapse model is introduced separately as an analytical approximation that reproduces the mean baryon-fraction shift, without evidence of the result being forced by construction or by self-citation chains. Self-references to the FLAMINGO framework or prior isocurvature implementations are not load-bearing for the central claims, which rest on the numerical comparison itself.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim relies on the validity of the FLAMINGO subgrid physics model, the correct implementation of isocurvature modes from linear theory, and the spherical collapse approximation for mean baryon fraction.

axioms (1)
  • domain assumption Standard Lambda CDM cosmology where baryon-CDM isocurvature perturbations arise naturally and freeze in amplitude at late times with RMS 1.5% on 10^11 solar mass halo scales.
    Invoked in the abstract to justify the existence and amplitude of the perturbations studied.

pith-pipeline@v0.9.0 · 5647 in / 1465 out tokens · 78311 ms · 2026-05-17T02:33:10.416560+00:00 · methodology

discussion (0)

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Works this paper leans on

1 extracted references · 1 canonical work pages · 1 internal anchor

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