Work

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

Year: 2026

Type: article

Abstract: ABSTRACT We use the FLAMINGO galaxy formation model to quantify the impact of baryon–cold dark matter (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 a root-mean-square amplitude of 1.5 per cent 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 anticorrelated with the matter overdensity field such that haloes form with baryon fractions below the cosmic mean, with earlier-collapsing haloes 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 haloes by $5~{{\ \rm per\ cent}}$ and $12~{{\ \rm per\ cent}}$, respectively, relative to the null-isocurvature case. These effects show no systematic dependence on halo mass and diminish steadily with time, reaching $0.1~{{\ \rm per\ cent}}$ and $1~{{\ \rm per\ cent}}$ 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 James Webb Space Telescope era.

Source: Monthly Notices of the Royal Astronomical Society

DOI: 10.1093/mnras/stag519

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

Institutions: Durham University, Leiden University, Leiden Observatory

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,046

Funders: European Commission, Durham University, Science and Technology Facilities Council, H2020 European Research Council