H II Galaxies versus Photoionization Models for Evolving Starbursts

We have constructed a grid of models representing an H II region produced by an evolving starburst embedded in a gas cloud of the same metallicity. The models were produced with the spectral energy distribution from a stellar evolutionary synthesis code as input for a photoionization code that computes the emission-line strengths and equivalent widths. Stellar evolution was assumed to proceed according to the models of Maeder. The radiation field was computed using the Kurucz model atmospheres, supplemented by the expanding non-LTE atmospheres of Schmutz et al. for stellar evolutionary phases with strong winds, making a significant improvement over previous works using classical static, plane-parallel model atmospheres. Models for stellar interiors and atmospheres being still in a phase of continuous improvement, our population synthesis models reflect the state of the art in 1995. The models were used to analyze a sample of 100 H II galaxies for which both the Hβ equivalent widths and the [O III] λ4363 line intensities were available (the latter allowing a direct determination of the oxygen abundances based on measured electron temperatures). Because of these selection criteria, the results of our study are restricted to metal-poor objects with metallicities less than about one-half solar. The confrontation of models with observations is presented in six diagnostic diagrams involving hydrogen and oxygen lines. Our approach is in many respects much more constraining for the models than previous studies on H II regions ionized by evolving starbursts. We found that the standard starburst model (instantaneous burst of star formation with a Salpeter initial mass function and an upper cutoff mass of 100 M☉) reproduces the observational constraints provided by the nebular emission lines extremely well if selection effects are taken into account. Models with a unique initial mass function are consistent with essentially all observational constraints over a metallicity range from ~0.025 to ~0.25 Z☉. In contrast, models with a Salpeter-type initial mass function truncated at 50 M☉ are not consistent with the observations: they violate the observed distribution of Hβ equivalent widths. The mean effective temperature of the ionizing star cluster declines from about 50,000 to 40,000 K during the time when the line [0 III] λ4363 is strong enough to be measurable. Within the framework of our models, and in the abundance range where comparisons were made with observations, there is no significant evidence for a variation of the star cluster mean effective temperature with metallicity, other than the one generated by the metallicity-dependent stellar atmospheric and evolutionary models. A very narrow range in ionization parameters is required to reproduce the observed line ratios. This should set limits on the dynamical evolution of giant H II regions. We find a large fraction of H II galaxies having [O I] λ6300/Hβ ratios larger than 0.02. Even models with the lowest ionization parameters considered do not produce these large ratios. An approximate estimate of the mechanical energy released by winds and supernovae during later phases of the starburst leads to the suggestion that the [O I] λ6300/Hβ ratio in contrast to other line ratios studied is significantly affected by shocks. The small spread in the free parameters necessary to reproduce the emission-line properties of metalpoor H II galaxies allows us to propose a new indicator of the starburst age: the [O III] λ5007 equivalent width is quite robust and can be used up to larger ages than the traditional Hβ equivalent width for high signal-to-noise spectra. This indicator should also prove useful for low signal-to-noise spectra of star-forming galaxies at higher redshift, because of the large value of [O III] λ5007/Hβ in starbursts younger than 5 Myr.