Characterising the formation and evolution of S0-galaxies (CaFES-0): Revealing the origin of the mass-size relation of S0 galaxies

We investigated the structural evolution and formation pathways of lenticular (S0) galaxies using the Hydrangea suite of cosmological hydrodynamical simulations. Simulated galaxies reproduce the observed mass–size relation from the SAMI and MaNGA surveys, enabling a direct comparison between morphology, angular momentum, and size growth. We show that the S0 population occupies a characteristic V-shaped locus in the mass–size plane, which arises from the superposition of two physically distinct formation channels. Low-mass S0s are predominantly faded-formed S0s that quenched after infall into their present-day host halo but retain the disk sizes of their star-forming progenitors. In contrast, high-mass S0s are merger-formed S0s that quenched before infall and exhibit structural properties and size evolution similar to those of ellipticals, consistent with pre-processing in group environments. By tracing their histories back to z = 1, we find that faded-formed S0s experience minimal structural evolution after quenching, whereas merger-formed S0s grow significantly in size through dissipationless interactions. These divergent evolutionary pathways explain both the slope break and the overall scatter of the S0 mass–size relation, demonstrating that lenticular galaxies arise from multiple formation mechanisms that leave distinct structural imprints.