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

We use the FLAMINGO galaxy formation model to quantify the impact of baryon-CDM isocurvature perturbations on galaxy formation in $\mathrm{\Lambda }$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 $1.5\mathrm{\%}$ on the Lagrangian scale of a ${10}^{11}{\mathrm{M}}_{\odot }$ halo ($0.85\mathrm{M}\mathrm{p}\mathrm{c}$). Although such perturbations arise naturally within $\mathrm{\Lambda }$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 $5\mathrm{\%}$ and $12\mathrm{\%}$, respectively, relative to the null-isocurvature case. These effects are almost independent of halo mass and diminish steadily with time, reaching $0.1\mathrm{\%}$ and $1\mathrm{\%}$ 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.