| Abstract
| - We have explored the emission line trends in the integrated spectra of normal spiral galaxies of the Nearby Field Galaxy Survey (Jansen et al. [CITE], b), in order to investigate the relationships between dust extinction, metallicity and some macroscopic properties of spiral galaxies. We found a very strong correlation between the H β and H α equivalent widths, implying that the difference between the extinction of the stellar and the nebular light depends only on the intrinsic colours of the galaxies, being larger for redder galaxies. The usual metallicity indicator for giant H ii regions ([O iii] λ4959,5007 + [O ii] λ3726,3729)/H β is not appropriate for integrated spectra of spiral galaxies, probably due to metallicity gradients. Much better qualitative metallicity indicators are found to be [N ii] λ6584/[O ii] λ3726,3729 and [N ii] λ6584/H α, the latter having the advantage of being independent of reddening and being applicable also for galaxies with weak emission lines. With these indicators, we find that the nebular extinction as derived from the Balmer decrement strongly correlates with the effective metallicity of the emission line regions. The overall metallicity of the emission line regions is much better correlated with galaxy colours than with morphological types. A Principal Component Analysis on a 7-D parameter space showed that the variance is produced, in first place, by the metallicity and parameters linked to the stellar populations, and, in second place, by the surface brightness, which is linked to the dynamical history of the galaxies. The absolute magnitude, related to the mass of the galaxy, comes only in third place.
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