Degeneracies between baryons and dark matter: the challenge of constraining the nature of dark matter with JWST

TL;DR

This study investigates how uncertainties in baryonic physics can mimic or obscure signals of different dark matter models in early galaxy formation, suggesting JWST may better constrain baryonic processes than dark matter properties.

Contribution

The paper demonstrates that baryonic physics uncertainties can degenerate with dark matter model predictions, complicating efforts to constrain dark matter nature using JWST data.

Findings

Differences in faint galaxy abundance between CDM and WDM are mimicked by baryonic physics variations.

Similar cosmic star formation histories can be achieved with different combinations of dark matter models and baryonic parameters.

JWST may be more effective in constraining baryonic physics than distinguishing dark matter models.

Abstract

The James Webb Space Telescope (JWST) will revolutionise our understanding of early galaxy formation, and could potentially set stringent constraints on the nature of dark matter. We use a semi-empirical model of galaxy formation to investigate the extent to which uncertainties in the implementation of baryonic physics may be degenerate with the predictions of two different models of dark matter -- Cold Dark Matter (CDM) and a 7 keV sterile neutrino, which behaves as Warm Dark Matter (WDM). Our models are calibrated to the observed UV luminosity function at $z=4$ using two separate dust attenuation prescriptions, which manifest as high and low star formation efficiency in low mass haloes. We find that while at fixed star formation efficiency, $\varepsilon$, there are marked differences in the abundance of faint galaxies in the two dark matter models at high-$z$, these differences are mimicked easily by varying $\varepsilon$ in the same dark matter model. We find that a high $\varepsilon$ WDM and a low $\varepsilon$ CDM model -- which provide equally good fits to the $z=4$ UV luminosity function -- exhibit nearly identical evolution in the cosmic stellar mass and star formation rate densities. We show that differences in the star formation rate at fixed stellar mass are larger for variations in $\varepsilon$ in a given dark matter model than they are between dark matter models; however, the scatter in star formation rates is larger between the two models than they are when varying $\varepsilon$. Our results suggest that JWST will likely be more informative in constraining baryonic processes operating in high-$z$ galaxies than it will be in constraining the nature of dark matter.