Standard BPT emission-line ratio diagram for HE 2158 − 0107. The measured line ratios of the NLR (filled red circle) and the EELR (opened red circle) are shown for the 3σ limits on log ( [N   ii] /Hα) and the length of the arrows are such that their tip corresponds to the 1σ limit. The distribution of emission-line ratios for  ~40 000 randomly selected SDSS galaxies are indicated by the grey scale for comparison. Triangle symbols indicate the NLR line ratio for a sample of AGN with low BH masses (Greene & Ho 2007) and blue opened squares represent four candidate low-metallicity AGN in dwarf galaxies as reported by Izotov & Thuan (2008) and Izotov et al. (2010).