BPT classification of the sample. Left: BPT diagram of the full sample. The dashed line is the demarcation between pure SF galaxies and AGN or composite galaxies according to Kauffmann et al. (2003). The solid line is the theoretical limit for the ionization state produced by star formation according to Kewley et al. (2001), splitting the sample further into AGN and composite systems. Right: distribution of stellar masses in the SF, AGN, composite and low-S/N subsamples.