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

Internal feedback from massive stars and active galactic nuclei (AGN) play a key role in galaxy evolution, but external environmental effects can also strongly influence galaxies. We investigate the impact of environment on galaxy evolution, and its dependence on baryonic physics implementation, using Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) spanning a wide range of stellar and AGN feedback implementations in the SIMBA, IllustrisTNG, ASTRID, and Swift-EAGLE galaxy formation models. We show that satellite galaxies are significantly affected by the environment in all simulation models, with their 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 ($f_{\rm B}$) and circumgalactic medium mass fraction ($f_{\rm CGM}$) at $z=0$ show clear environmental effects. In SIMBA, low-mass haloes in overdense regions have systematically lower $f_{\rm B}$ and $f_{\rm CGM}$ at fixed halo mass, while Swift-EAGLE haloes in overdense regions have systematically higher $f_{\rm B}$ and $f_{\rm CGM}$ 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 $f_{\rm B}$ increasing with local density in low-mass haloes before quenching at an increasing overdensity threshold. Our results demonstrate that the impact of environment on galaxy evolution depends significantly on galaxy formation model, and higher-density environments can either suppress or enhance star formation depending on galaxy mass and cosmic epoch.