Orbital decomposition and evolution of LEGA-C -like galaxies in IllustrisTNG from z=1 to z=0

Stellar orbits encode the dynamical history of galaxies and provide a physically motivated link between galaxy structures and assembly processes. Although orbit-based decompositions are well established in the Local Universe, their evolution across cosmic time, especially at higher redshifts, remains poorly explored. We investigate the orbital structure of galaxies in the simulation from z=1 to z=0 to identify how stellar mass, star formation activity, environment, merger history, gas content, and ex situ stellar mass growth shape their orbital transformation. LEGA-C-like TNG100 We decomposed the stellar orbital structure of simulated galaxies into cold, warm, hot, and counter-rotating (CR) components using time-averaged orbital circularities. This provides a theoretical basis for future comparisons with Schwarzschild-based orbital decompositions of observed galaxies. We analyzed the galaxy populations at z=1 and z=0, then tracked the descendants of individual z=1 galaxies to quantify their orbital evolution over this time. We find a strong mass dependence of orbital structure at z=0 as well as at z=1. Galaxies below łog(M_⋆/M_⊙) have a combined warm and cold orbital fraction of sim70%, whereas more massive systems have a hot-orbit fraction of sim60%. These trends are consistent with assembly pathways driven predominantly by wet and dry major mergers, respectively. Tracking the same galaxies to z=0 reveals a systematic, but mass-dependent, evolution toward lower rotational support: At both low and high stellar masses, the hot-orbit fraction increases, whereas the relative change is greater in low-mass galaxies. The survival of cold structure depends strongly on the star formation state at z=1; star-forming progenitors retain substantially larger cold fractions than the quiescent progenitors. At both redshifts, the orbital fractions show a rise (decline) in hot and CR (cold and warm) orbits with the ex situ stellar mass fraction at a fixed stellar mass. These results provide the first evolutionary framework for interpreting orbit-based decompositions of galaxies at the epoch. Stellar orbital fractions quantify the morphological and dynamical transformation of galaxies and show systematic evolutionary trends at the population level. Individual galaxies, however, follow diverse evolutionary pathways. Comparisons to future Schwarzschild-based decompositions of observed high-redshift galaxies can use these results to interpret the internal physical structure of galaxy populations and constrain theoretical models of galaxy evolution. LEGA-C