CAMELS Environments: The Impact of Local Neighbours on Galaxy Evolution across the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE Simulations

Abstract

Internal feedback from massive stars and active galactic nuclei play a key role in galaxy evolution, but external environmental effects can also strongly influence galaxies. We investigate the impact of environment and its dependence on baryonic physics implementation using Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) spanning a wide range of feedback implementations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE models. We show that satellite galaxies are significantly affected by environment in all models, with gas fraction and star formation rate (SFR) suppressed in overdense regions compared to similar mass satellites in underdense environments at z = 0. Central galaxies are less sensitive to environment but tend to show lower gas fraction and SFR in overdense regions at low stellar mass, transitioning to higher gas fraction and SFR for massive galaxies in higher-density environments. Halo baryon fraction (fB) and circumgalactic medium mass fraction (fCGM) at z = 0 show clear environmental effects. In SIMBA, low-mass haloes in overdense regions have systematically lower fB and fCGM at fixed halo mass, while Swift-EAGLE haloes in overdense regions have higher fB and fCGM across the full halo mass range, and IllustrisTNG and ASTRID show opposite trends at the low and high mass ends. Environmental effects can flip at higher redshift, with SFR and fB increasing with local density in low-mass haloes before quenching at an increasing overdensity threshold. Our results demonstrate that the impact of environment depends significantly on galaxy formation model, and higher-density environments can either suppress or enhance star formation depending on galaxy mass and cosmic epoch.