Cosmological back-reaction of baryons on dark matter in the CAMELS simulations

ABSTRACT Baryonic processes such as radiative cooling and feedback from massive stars and active galactic nuclei directly redistribute baryons in the Universe but also indirectly redistribute dark matter due to changes in the gravitational potential. In this work, we investigate this ‘back-reaction’ of baryons on dark matter using thousands of cosmological hydrodynamic simulations from the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project, including parameter variations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE galaxy formation models. Matching haloes to corresponding N-body (dark matter-only) simulations, we find that virial masses decrease owing to the ejection of baryons by feedback. Relative to N-body simulations, halo profiles show an increased dark matter density in the centre (due to radiative cooling) and a decrease in density farther out (due to feedback), with both effects being strongest in SIMBA ($\gtrsim$ 450 per cent increase at $r \lesssim 0.01\, R_{\rm vir}$). The clustering of dark matter strongly responds to changes in baryonic physics, with dark matter power spectra in some simulations from each model showing as much as 20 per cent suppression or increase in power at $k \sim 10\, h$ Mpc$^{-1}$ relative to N-body simulations. We find that the dark matter back-reaction depends intrinsically on cosmology ($\Omega _{\rm m}$ and $\sigma _{8}$) at fixed baryonic physics, and varies strongly with the details of the feedback implementation. These results emphasize the need for marginalizing over uncertainties in baryonic physics to extract cosmological information from weak lensing surveys as well as their potential to constrain feedback models in galaxy evolution.