The 5007/H and 6583/H ratios are strongly correlated in H II regions. Theoretical studies show that the heavy-metal abundances change continuously along this sequence, a low 5007/H ratio indicating a high metal abundance and a high 5007/H ratio, a low metal abundance, with the heavy metal abundances changing from 1.5 Z at the lower right of Fig. a to 0.25 Z at the upper left (see for instance Dopita & Evans 1986; Ho et al. 1997b). However, Stasinska & Leitherer (1996) have shown that most startbusts and H II galaxies can be described as being produced by an evolving starburst with an universal initial mass function embedded in a gas cloud of the same metallicity. The emission line ratios depend mainly on two independent parameters: the age of the starburst and the metallicity. In this scenario, the 5007/H ratio effectively changes with these two parameters and therefore is not a direct measurement of metallicity. The metallicity is strongly correlated with luminosity, luminous galaxies having higher metallicities; this correlation is also valid for elliptical galaxies, for which the metallicity is determined from absorption lines with [O/H] 1 at 21 (Salzer et al. 1989; Zaritsky et al. 1994).
AGNs are known to occur preferentially in high luminosity (Ho et al. 1997b), early-type (Véron & Véron-Cetty 1986; Vacali et al. 1997) galaxies; they are therefore expected to have high metallicities. Indeed, the NLRs of active galactic nuclei have enhanced nitrogen abundances (Storchi-Bergmann & Pastoriza 1989, 1990; Storchi-Bergmann et al. 1992; Schmitt et al. 1994). In these NLRs, [N/O] correlates with [O/H] in a manner identical to H II regions in normal galaxies, with nuclear [O/H] and [N/O] values ranging from 1 Z to 2 Z (Storchi-Bergmann et al. 1996b). Storchi-Bergmann et al. (1996b,c) have determined the chemical composition of the H II regions in the ring surrounding the nucleus of several AGNs, as well as in the nuclei; high metallicities were found ([O/H] 2 Z and [N/O] 3 Z) both in the H II regions and in the AGNs, these abundances being similar to those found in the nuclei of non-active galaxies with the same morphological type and absolute magnitude. Further work by Storchi-Bergmann et al. (1998) has shown that, in fact, oxygen abundances derived for Seyfert 2 nebulosities and neighbouring H II regions (assuming that the emission lines in the active nucleus are due to photoionization by a typical active galactic nucleus continuum) are well correlated, while this is not the case for Liners. This suggests that the gas in AGNs and in the neighbouring H II regions has the same origin and that the scatter observed in the Seyfert 2 region in the diagnostic diagrams, involving the 6583/H ratio, is due to variations in the nitrogen abundance. In NGC 6300, in which 6583/H = 3.4, the nitrogen abundance is estimated to be 5 Z.
We have seen that nuclear H II regions and Seyfert 2 nebulosities, when appearing in the same galaxy, have the same high metallicity; as a result of their metallicity, the H II regions have a low excitation, while the Seyfert 2 nebulosities have a high excitation. This explains why it is relatively easy to separate the two components in ``transition'' spectra.