The BPT (Baldwin et al. 1981) diagnostic diagram used to classify the spectra of galaxies in this work, based on the [Nii]/Hα and [Oiii]/Hβ line ratios. Objects for which a measurement of [Oiii]/Hβ is not available are plotted at log([Oiii]/Hβ) = −1.3. The Decarli et al. (2007) scheme (straight dashed lines) marks the classification boundaries between star-forming, LINER, Seyfert and composite/transition galaxies. In addition, the demarcation line between star-forming and AGN-host galaxies from Kauffmann et al. (2003, solid black line) is shown. Potential AGN-host galaxies (red open circles) are thus removed from the sample of normal star-forming systems (blue closed circles).
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