Gal3D: Superellipsoid Modeling of Radial 3D Galaxy Structure in IllustrisTNG and EAGLE Simulations

Abstract

Galaxy morphology and structure are key tracers of galaxy formation and evolution, making accurate measurements of intrinsic three-dimensional (3D) shape essential for linking morphology to galaxy assembly and for comparing numerical simulations. We present Gal3D, a framework that reconstructs smoothed density fields from particle data and quantifies the radial 3D structure of simulated galaxies by fitting superellipsoids to iso-density surfaces. The method recovers axis ratios, orientations, center offsets, and superellipsoid indices ($S_a$, $S_b$, $S_c$), enabling a flexible characterization of diverse galactic structures such as disks, classical bulges, box/peanut bulges, and triaxial components. Applying Gal3D to galaxies in the IllustrisTNG and EAGLE simulations, we find that the radial extent of flattened disk regions increases with stellar mass up to $M_{*,30}\sim10^{11}\,M_\odot$ and then declines sharply, with EAGLE galaxies showing a saturation at $M_{*,30}\sim10^{10.5}\,M_\odot$. The bar-related $ \varepsilon_{ab}\equiv 1-b/a$ strengthens above $M_{*,30}\sim10^{10.5}\,M_\odot$ in both simulations, but remains systematically weaker in EAGLE. In TNG, outer bar regions are commonly associated with elevated $S_a$ and $S_c$, indicating enhanced boxiness and more prominent box/peanut-shaped bulges, whereas such higher-order signatures are weak or absent in EAGLE. At the highest stellar masses, flattened disks become less prominent, while inner prolate or triaxial structures remain common and massive EAGLE galaxies have more prolate or triaxial outer stellar bodies than their TNG counterparts. These results demonstrate that Gal3D provides a practical framework for quantifying intrinsic radial 3D structure and comparing morphology across cosmological simulations.