Modeling the emission line sequence of HII galaxies

Using a sample of unprecedented size (about 400 objects) of H II galaxies in which the oxygen abundances have been obtained using the temperature derived from the [O III] lambda4363/5007 line ratio, we confirm that the H II galaxies form a very narrow sequence in many diagrams relating line ratios and Hbeta equivalent width. We divide our sample in three metallicity bins, each of which is compared with sequences of photoionization models for evolving starbursts with corresponding metallicity. Our aim is to find under what conditions a theoretical sequence can reproduce all the observed trends. Taking into account the presence of an older, non-ionizing stellar population, for which independent indications exist, we find that the simple model of an adiabatic expanding bubble reproduces the observational diagrams very well if account is taken of an aperture correction and the covering factor is assumed to decrease with time exponentially with an e-folding time of 3 Myr. We find that the He II lambda4686 nebular line emission occurs too frequently and in too wide a range of EW(Hbeta) to be attributable to either the hard radiation field from Wolf-Rayet stars or the X-rays produced by the latest stellar generation. Assuming that the He II lambda4686 line is due to photoionization by a hot plasma at a temperature of 10 6 K, a total X-ray luminosity of 10(40) 4 x 10(40) erg s(-1) is required for at least half of the sources. We find evidence for self-enrichment in nitrogen on a time scale of several Myr, and argue for a possible self-enrichment in oxygen as well.