2.1 H II regions
Abundances are relatively easy to measure in star-forming dwarf galaxies because they contain gas clouds in which large numbers of hot stars are embedded. Their spectra are dominated by nebular emission lines similar to those of high-excitation giant H II regions in late type spiral galaxies. What is observed in the optical are narrow emission lines superimposed on a blue stellar continuum (see Fig 6.). They are identified as helium and hydrogen recombination lines and several forbidden lines. Methods used in determining abundances are well understood and generally more reliable than those based on stellar absorption line data because transfer problems become of minor importance. From the optical, O, N, S, Ne, Ar and He lines are currently measured. With modern detectors fainter lines such as lines of Fe have been studied (Izotov and Thuan, 1999). The ultraviolet (UV) region is dominated by the hot stellar continuum and shows relatively weak emission lines except for those that originate in stellar winds. However there are a few notable exceptions and owing to the International Ultraviolet Explorer (IUE) and more recently the Hubble Space Telescope (HST) nebular carbon and silicon abundances have been determined.
Oxygen is the most reliably determined element, since the most important ionisation stages can all be observed. Moreover the [O III] 4363 line allows an accurate determination of the electron temperature. The intrinsic uncertainty in this method (reflecting a simplified conception of the H II region physics, possible problems with temperature fluctuations etc.) is of the order of ~ 0.1 dex (Pagel 1997). Furthermore, when the electron temperature cannot be determined, empirical relations (cf. Pagel 1997) between the oxygen abundance and the [O II]3727 and [O III]4959, 5007 strength relative to H are used, though with lower accuracy (0.2 dex or worse). For other species, in general, one does not observe all the ionisation stages expected to be present in the photoionisation region and an ionisation correction factor must be applied to derive the total abundance of the element in question.
One important aspect of H II region abundances is that they can be obtained also at great distances. This makes them powerful tools also for studying high redshift galaxies, with the price that our view will be biased towards actively star-forming systems. For a discussion on possible problems associated with deriving abundances in very distant galaxies, see Kobulnicky et al. (1999).