Emission-line diagnostic diagram for the optical nucleus spectrum of each galaxy in the sample. Only objects for which all required emission lines have signal-to-noise larger than three are shown (166 galaxies). The demarcation lines of Kewley et al. (2001) (red curve), Kauffmann et al. (2003) (dashed-blue curve), and Cid Fernandes et al. (2010) (dotted-green straight line) are used to classify the galaxies into star forming (SF), active galactic nuclei (AGN), and LINER-type galaxies, which are denoted with black, red, and green symbols, respectively.