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 H 11 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 [O III]/H beta, [O II]/[O III], [S II]/H alpha and [N II]/[S II] 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 H alpha and [O II] luminosities produced per unit star formation rate (SFR). When accounting for stellar H alpha absorption and absorption by dust in the neutral ISM, the actual uncertainties in SFR estimates based on the emergent H alpha and [O 11] luminosities are as high as several decades. We derive new estimators of the SFR, the gas-phase oxygen abundance and the effective absorption optical depth of the dust in galaxies. We show that, with the help of other lines such. as [O III], H beta, [O III], [N II] or [S II], the uncertainties in SFR estimates based on H alpha can be reduced to a factor of only 2-3, even if the H alpha line is blended with the adjacent [NII] lines. Without H alpha, however, the SFR is difficult to estimate from the [O II], H beta and [O III] lines. The reason for this is that the absorption by dust in the neutral ISM and the ionized-gas parameters are then difficult to constrain independently This suggests that, while insufficient by itself, the H alpha line is essential for estimating the star formation rate from the optical emission of a galaxy.
