The formation and evolution of galaxies over cosmic time is one of the most challenging and lively fields in astrophysics today. Recent advancements with the JWST and ALMA have revealed the presence of strong ionization sources such as massive star formation and active galactic nuclei (AGN) within galaxies in the early Universe through observations of ionized gas emission lines. However, the relative contributions of star formation and AGN to gas ionization in early galaxies are not well understood. To interpret these observations, it is crucial to use cosmological simulations that incorporate small-scale physics, as the observed emission lines originate from neutral gas clouds and ionized regions (<10 pc). In this project, I will develop a new suite of high-resolution radiation hydrodynamic simulations for early galaxies that include essential small-scale physical processes. Furthermore, I will generate synthetic observations for stellar and dust emission, and for ionized atomic lines by applying post-processing radiative transfer on the simulations. Finally, I will disentangle the contributions of AGN and star formation to gas ionization in the early galaxies by analyzing the physical origins of the ionized gas emission lines in the simulated galaxies.