Astron. Astrophys. 321, 29-44 (1997)

Effects of star cluster evolution on emission line ratios and the application to H II galaxies

Including a calibration of the R₂₃ vs. 12+log(O/H) relationship

K. Olofsson

Astronomiska observatoriet, Box 515, S-751 20 Uppsala, Sweden

Received 20 December 1994 / Accepted 22 July 1996

Abstract

A model of spectral evolution of star forming galaxies of various metallicity has been used to study the relation between the optical emission line ratio R₂₃ = ([OII] 3727 +[OIII] 4959,5007)/H and the nebular oxygen abundance. It is shown that the scatter in an empirical comparison sample could be due to various upper stellar mass limits or different slopes of the stellar initial mass function in these objects. However, it is more likely to be an effect of evolution of the underlying stellar population, i.e. the star forming knots observed are of different ages, or equally, the temperature of the ionizing radiation is different. This is particularly evident studying the evolution a few million years after the star formation has ceased. These calculations show that the so called lower branch of the relation is much less sensitive to age than the turn-over region or the upper branch using an instantaneous burst of star formation. This is because massive stars, which dominate the light in a young stellar population, of low metallicity remain at higher effective temperature compared to a star of higher metallicity. In a region of active star formation the R₂₃ vs. oxygen abundance relation is even more pronounced since it proves to be very insensitive to age. In the absence of a temperature-sensitive line for the oxygen abundance determination an empirical or semi-empirical approach is normally utilized relying on a calibration of the R₂₃ vs. oxygen abundance relation. In this analysis a new calibration of the upper branch, based on a fit of theoretical curves to empirical data of the lower branch, is attempted.

Key words: galaxies: evolution galaxies: stellar content galaxies: starburst

Contents

• 1. Introduction
• 2. The theoretical basis
  • 2.1. The model of spectral evolution
  • 2.2. Stellar evolutionary tracks
  • 2.3. The library of model stellar spectra
  • 2.4. The nebular component
  • 2.5. Miscellaneous
• 3. Results and discussion
  • 3.1. The empirical comparison sample
  • 3.2. The length of the burst of star formation
  • 3.3. Variations in the IMF
  • 3.4. Calibration of the R₂₃ vs. oxygen abundance relation
    • 3.4.1. The theoretical age vs. relation
    • 3.4.2. The of the empirical sample
    • 3.4.3. The oxygen abundance calibration
• 4. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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