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

Abstract We use the FLAMINGO galaxy formation model to quantify the impact of baryon-CDM isocurvature perturbations on galaxy formation in ΛCDM. 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 ΛCDM, 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 show no systematic dependence on 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.