Down-bending Breaks in Galactic Disks Are an Intrinsic By-product of Inside-out Growth

Abstract The exponential profile has long been hypothesized as the fundamental morphology of galactic disks. The IllustrisTNG simulations successfully reproduce diverse surface-density profiles: type I (single exponential), type II (down-bending), and type III (up-bending), matching reasonably well with observed mass–size relations and kinematics. Type II disks dominate the stellar mass regime M ⋆ < 10 10.6 M ⊙ with a prevalence of ∼40%, exhibiting systematically extended morphologies. Conversely, type III and type I galaxies occupy more compact configurations, following the same mass–size scaling relation. Evolutionary history analysis shows that type II galaxies experience minimal external perturbation, representing the intrinsic disk form—challenging conventional single-exponential paradigms. We demonstrate that type II breaks arise naturally via inside-out growth during z < 1, governed by synchronized cold-gas accretion and localized specific star formation rate peaks. This mechanism also produces the characteristic U-shaped age profiles of type II disks. Crucially, dynamical redistribution of stars plays an unimportant role in their formation.