17 - Measuring chemical abundances in extragalactic metal-rich H ii regions

Summary

Chemical abundances of metal-rich H II regions: why?

Ionized nebulae (H II regions) trace the sites of massive-star formation in spiral and irregular galaxies. The rapid evolution of these stars, ending in supernova explosions, and the subsequent recycling of nucleosynthesis products into the interstellar medium, make H II regions essential probes of the present-day chemical composition of star-forming galaxies across the Universe. The study of nebular abundances is therefore crucial for understanding the chemical evolution of galaxies. In the following pages I will provide an optical astronomer's perspective on some of the issues concerning the measurement of abundances in metal-rich H II regions, by focusing on the observational difficulties that are peculiar to the high-metallicity regime, discussing some of the most recent abundance determinations from H II regions in the metal-rich zones of spiral galaxies, and indicating some possibilities for further progress. Throughout this paper I will use the oxygen abundance as a proxy for the metallicity (oxygen makes up roughly half of the metal content of the interstellar medium), and assume the Solar value from Asplund et al. (2004), 12 + log(O/H)_⊙ = 8.66. Elements besides oxygen will not be discussed in great detail.

Motivations

Why measure abundances of metal-rich H II regions? After all, as we will see in Section 2, metal-rich H II regions pose difficulties to the observer that are not present at lower metallicities, i.e. roughly below half the Solar O/H value. However, high abundances are encountered in a variety of astrophysical contexts, and the study of ionized nebulae often provides the only way to measure these abundances.