Spectral classification of emission-line galaxies

The main goal of this work is to further investigate the classification of emission-line galaxies from the "Spectrophotometric Catalogue of H II galax ies" by Terlevich et al. (1991) in a homogeneous and objective way, using t he three line-ratio diagrams, called diagnostic diagrams, of Veilleux & Ost erbrock (1987). On the basis of the resulting catalogue, we critically discuss the classifi cation methods in the optical range. In particular we compare our classific ation scheme to the one done by Rola et al. (1997) which is efficient for t he classification of redshifted galaxies. We also propose a new diagnostic diagram involving the known intensity ratio R-23=([O II] lambda 3727+[O III ] lambda 4959+[O III] lambda 5007)/H beta which appears to be a very good c riterion allowing to discriminate the Seyfert 2 from H II galaxies. The revised catalogue including 314 narrow-emission-line galaxies(1) contai ns H II galaxies, Seyfert 2 galaxies, Low Ionization Nuclear Emission-Line Regions (hereafter LINERs) galaxies and some particular types of galaxies w ith the most intriguing ones, called "ambiguous", due to the ambiguity of t heir location in the diagnostic diagrams. These galaxies appear as H II gal axies and as active galactic nuclei (hereafter AGNs) in different diagrams of Veilleux & Osterbrock and constitute certainly a sample of particularly interesting candidates for a thorough study of connections between starburs ts and AGNs.