2001A&A...365..347M

We analyze coherently the stellar and nebular energy distributions of starbursts and HII galaxies, using our evolutionary synthesis model, PEGASE (Fioc & Rocca-Volmerange 1997A&A...326..950F, 2000, in prep.), coupled to the photoionization code CLOUDY (Ferland, 1996, HAZY, a brief introduction to Cloudy, University of Kentucky, Dept. of Physics and Astromomy Internal Report). The originality of this study is to relate the evolution and the metallicity of the starburst to the past star formation history of the host galaxy. Extinction and geometrical effects on emission lines and continua are computed in coherency with metallicity. We compare our model predictions to an observed sample of ≃750 HII regions and starbursts. When fitting [OIII]_λ4363/[OIII]_λ5007, [OI]_λ6300/Hα, [SII]_λλ6716, 6731_/Hα, [NII]_λ6584/Hα and [OIII]_λ5007/Hβ, the most striking feature is the decreasing spread in U with increasing metallicity Z. High-U objects systematically have a low metallicity while low levels of excitation happen at any Z. The best fits of emission line ratios are obtained with a combination of a high- and a low-ionization components. No additional source of ionizing photons - shocks or hidden AGN - is needed. The high level of excitation observed in metal-poor HII galaxies requires a very young population (≤3Myr), while starburst nuclear galaxies (SBNGs) are consistent with a wider range of age (≤5Myr). Colors (B-V, V-R) and equivalent widths are fitted in coherency with emission line ratios. An underlying population is needed, even for small-aperture observations. This underlying population not only reddens the continuum and dilutes the equivalent width of the emission lines, but also participates in the ionization process. Its main effect on line ratios is to maintain a high level of excitation when the burst stops. Models combining underlying populations typical of Hubble sequence galaxies and instantaneous starbursts with ages between 0 and 8Myr agree satisfactorily with all the data.