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Ripples in the baryon to dark matter ratio in $Λ$CDM: implications for galaxy formation

Owen Jessop, Adrian Jenkins, Andrew Pontzen, Joop Schaye, Matthieu Schaller, John C. Helly

TL;DR

This work investigates how baryon–CDM isocurvature perturbations, which are local, compensated variations in the baryon-to-CDM ratio with a late-time amplitude of about $1.5\%$, influence galaxy formation in a $\Lambda$CDM context. Using three FLAMINGO hydrodynamical runs with identical matter overdensity fields but different isocurvature realizations, the authors show that isocurvature can modestly suppress halo gas content and star formation at $z\approx8$ (by about $5\%$ and $12\%$, respectively) and delay BH growth (up to $\sim15\%$ in black hole mass density by $z\sim5$), with effects diminishing to $\lesssim1\%$ by $z=0$. A simple spherical-collapse model connects the isocurvature amplitude to changes in the initial baryon fraction, and the simulations largely agree with this analytic baseline. The findings imply that high-redshift predictions from simulations should include isocurvature ICs to improve fidelity, particularly for JWST-era observations, though the overall impact is subdominant to existing modeling uncertainties at late times.

Abstract

We use the FLAMINGO galaxy formation model to quantify the impact of baryon-CDM isocurvature perturbations on galaxy formation in $Λ$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 $Λ$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.

Ripples in the baryon to dark matter ratio in $Λ$CDM: implications for galaxy formation

TL;DR

This work investigates how baryon–CDM isocurvature perturbations, which are local, compensated variations in the baryon-to-CDM ratio with a late-time amplitude of about , influence galaxy formation in a CDM context. Using three FLAMINGO hydrodynamical runs with identical matter overdensity fields but different isocurvature realizations, the authors show that isocurvature can modestly suppress halo gas content and star formation at (by about and , respectively) and delay BH growth (up to in black hole mass density by ), with effects diminishing to by . A simple spherical-collapse model connects the isocurvature amplitude to changes in the initial baryon fraction, and the simulations largely agree with this analytic baseline. The findings imply that high-redshift predictions from simulations should include isocurvature ICs to improve fidelity, particularly for JWST-era observations, though the overall impact is subdominant to existing modeling uncertainties at late times.

Abstract

We use the FLAMINGO galaxy formation model to quantify the impact of baryon-CDM isocurvature perturbations on galaxy formation in 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 on the Lagrangian scale of a halo (). Although such perturbations arise naturally within 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 , isocurvature perturbations reduce the mean baryon fraction and star formation rates of resolved halos by and , respectively, relative to the null-isocurvature case. These effects are almost independent of halo mass and diminish steadily with time, reaching and by . 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.

Paper Structure

This paper contains 15 sections, 20 equations, 3 figures.

Table of Contents

  1. Introduction
  2. Baryon--CDM isocurvature perturbations in $\Lambda$CDM
  3. Decomposition of the isocurvature overdensity field
  4. Predicting the effects of isocurvature perturbations on halo formation
  5. The Simulations
  6. The FLAMINGO galaxy formation model
  7. The initial conditions
  8. Structure & substructure identification
  9. Results
  10. The time evolution of the baryon and gas content of halos
  11. The time evolution of stellar mass
  12. The time evolution of black hole mass
  13. The time evolution of the mean halo concentration
  14. Discussion & Conclusions
  15. The time evolution of Baryons and CDM in an Einstein de-Sitter Universe after Baryon--Photon Decoupling

Figures (3)

  • Figure 1: $z=0$ linear density fields computed with CLASS for the matter overdensity field, $\delta_{\rm m}$, the isocurvature overdensity field, $\delta_{\rm{bc}}$, and the orthogonal isocurvature overdensity field, $\delta_{\rm{bc}}^{\perp}$, see \ref{['eq:delta_m1', 'eq:delta_bc2', 'eq:decomp3']}. The fields have been smoothed with a spherical top-hat filter on the Lagrangian scale corresponding to a $10^{11}\,\rm{M_\odot}$ halo ($R =0.85\,\rm{Mpc}$). The cosmological parameters used correspond to the fiducial cosmology adopted throughout this paper (see \ref{['subsec:flamingo_model']}). We sample the fields on a periodic $500\,\rm{Mpc}$ comoving cubic volume with $900^3$ voxels; we display the central $250\,\rm{Mpc}$ region. Each panel shows an $x$–$y$ slice through the box centre with the root-mean-square (RMS) amplitude, $\sigma_{\rm{RMS}}$, given in the upper left corner of each panel.
  • Figure 2: Top panel: The time evolution of the comoving baryon mass density in resolved halos, $\rho_{\rm b}^{\rm halos}(z)$, 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 \ref{['subsec:linear_collapse']}, \ref{['eq:final model']}.
  • 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 \ref{['subsec:linear_collapse']}, \ref{['eq:final model']}. Jackknife errors are computed for each simulation, propagated through to the ratios, and shown as shaded regions. Panel (a): the comoving gas mass density in resolved halos, $\rho_{\rm gas}^{\rm halos}(z)$; Panel (b): the comoving star formation rate density, $\dot{\rho}_{\rm star}(z)$; Panel (c): the comoving stellar mass density, $\rho_{\rm star}(z)$; Panel (d): the comoving BH accretion rate density, $\dot{\rho}_{\rm{BH}}(z)$; Panel (e): the comoving BH mass density, $\rho_{\rm{BH}}(z)$; Panel (f): The mean halo concentration, $\langle c\rangle(z)$.