2023ApJ...953..140N

Recent observations by the James Webb Space Telescope discovered a number of high-redshift galaxies with strong emission lines from doubly ionized oxygen. Combined with Atacama Large Millimeter/submillimeter Array observations of far-infrared lines, multiline diagnostics can be applied to the high-redshift galaxies in order to probe the physical conditions of the interstellar medium. We study the formation and evolution of galaxies using the FirstLight simulation suite, which provides outputs of 62 high-resolution, zoom-in galaxy simulations. We devise a physical model of H II regions and calculate spatially resolved [O III] line emission. We show that massive galaxies with stellar masses of M_* > 10⁹M_⊙ chemically evolve rapidly to z = 9. Young stellar populations in the star-forming galaxies boost the [O III] line emission, rendering the ratio of line luminosity to star formation rate larger than that for low-redshift galaxies, which is consistent with recent observations. Measuring the flux ratios of rest-frame optical and far-infrared lines allows us to estimate the physical conditions such as density and metallicity of the star-forming gas in high-redshift [O III] emitters.