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We present a new model for computing consistently the line and continuum emission from galaxies, based on a combination of recent population synthesis and photoionization codes. We use effective parameters to describe the Hii regions and the diffuse gas ionized by single stellar generations in a galaxy, among which the most important ones are the zero-age effective ionization parameter, the effective gas metallicity, and the effective dust-to-heavy element ratio. We calibrate the nebular properties of our model using the observed Oiii/Hβ, Oii/Oiii, Sii/Hα, and Nii/Sii ratios of a representative sample of nearby spiral and irregular, starburst, and Hii galaxies. To compute whole (line plus continuum) spectral energy distributions, we include the absorption by dust in the neutral interstellar medium (ISM) using a recent simple prescription, which is consistent with observations of nearby starburst galaxies. Our model enables us to interpret quantitatively the observed optical spectra of galaxies in terms of stars, gas, and dust parameters. We find that the range of ionized-gas properties spanned by nearby galaxies implies factors of 3.5 and 14 variations in the Hα
Charlot et al. (Wed,) studied this question.
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