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

TL;DR

This study quantifies how baryon-CDM isocurvature perturbations influence galaxy formation in $ m f extLambda$CDM, showing they reduce baryon fractions and star formation rates at high redshift, with effects diminishing over time.

Contribution

It demonstrates the significance of including baryon-CDM isocurvature modes in galaxy formation models, which are often neglected in current simulations.

Findings

Isocurvature perturbations reduce baryon fractions by 5% at $z=8$

Star formation rates decrease by 12% at $z=8$ due to these perturbations

Effects diminish to near zero by $z=0$

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.