Euclid: An emulator for baryonic effects on the matter bispectrum★

The Euclid mission and other next-generation large-scale structure surveys will enable high-precision measurements of the cosmic matter distribution. Understanding the impact of baryonic processes such as star formation and active galactic nuclei (AGN) feedback on matter clustering is crucialto ensure precise and unbiased cosmological inference. Most theoretical models of baryonic effects to date focus on two-point statistics, neglectinghigher-order contributions. This work develops a fast and accurate emulator for baryonic effects on the matter bispectrum, a key non-Gaussianstatistic in the non-linear regime. We employ high-resolution 𝑁-body simulations from the BACCO suite and apply a combination of cutting-edgetechniques such as cosmology scaling and baryonification to efficiently span a large cosmological and astrophysical parameter space. A deep neuralnetwork is trained to emulate baryonic effects on the matter bispectrum measured in simulations, capturing modifications across various scalesand redshifts relevant to Euclid. We validate the emulator accuracy and robustness using an analysis of Euclid mock data, employing predictionsfrom the state-of-the-art FLAMINGO hydrodynamical simulations. The emulator reproduces baryonic suppression in the bispectrum to better than2% for the 68% percentile across most triangle configurations for 𝑘 ∈ [0.01, 20] ℎ/Mpc and ensures consistency between cosmological posteriorsinferred from second- and third-order weak lensing statistics. These results demonstrate that our emulator meets the high-precision requirementsof the Euclid mission for at least the first data release and provides forecasts of the cosmological and astrophysical information contained in thesmall-scale matter bispectrum. This underscores the potential of emulation techniques to bridge the gap between complex baryonic physics andobservational data, maximising the scientific output of Euclid.Key words. Gravitational lensing: weak – Surveys – cosmological parameters – large-scale structure of Universe