The evolution of emission lines in HII galaxies

G. Stasinska et al., The evolution of emission lines in HII galaxies, ASTRON ASTR, 370(1), 2001, pp. 1-22
Citations number
Categorie Soggetti
Space Sciences
Journal title
ISSN journal
0004-6361 → ACNP
Year of publication
1 - 22
SICI code
We constructed diagnostic diagrams using emission line ratios and equivalen t widths observed in several independent samples of H II galaxies. Signific ant treads are seen, both in the line ratio diagrams, and in diagrams relat ing line ratios to the equivalent width of HP. The diagrams are compared to predictions from photoionization models fur evolving starbursts. This stud y extends the work of Stasinska & Leitherer (1996) by including objects wit h no direct determination of the metallicities, and by using updated synthe sis models with more recent stellar tracks and atmospheres. We find that H II galaxies from objective-prism surveys are not satisfactorily reproduced by simple models of instantaneous starbursts surrounded by constant density , ionization bounded I-I Ir regions. The observed relations between emissio n line ratios and HP equivalent width can be understood if older stellar po pulations generally contribute to the observed optical continuum in H II ga laxies. In addition, different dust obscuration for stars and gas and leaka ge of Lyman continuum photons from the observed H II regions can be importa nt. As a result;, H II galaxies selected from objective-prism surveys are n ot likely to contain significant numbers of objects in which the most recen t starburst is older than about 5 Myr. This explains the success of the str ong line method to derive oxygen abundances, at least in metal poor H Ir ga laxies. The observed increase of [O I]/H beta with decreasing H beta equiva lent width can result from the dynamical effects of winds and supernovae. T his interpretation provides at the same time a natural explanation of the s mall range of ionization parameters in giant H II regions. The classical di agnostic diagram [O III]/H beta vs. [O II]/H beta cannot be fully understoo d in terms of pure photoionizaition models. The largest observed [O II]/H b eta ratios require additional heating. The [N II]/(O II] ratio is shown to increase as the HP equivalent width decreases. A possible explanation is an N/O increase due to gradual enrichment. by winds from Wolf-Rayet stars on a time scale of similar to5 Myr Alternatively, the relation between N/O and O/H could be steeper than N/O proportional to O/H-0.5, with a previous ste llar generation more important at higher metallicities.