Prediction from our Cloudy models of the emission line ratios [O III]/Hβ vs [N II]/Hα (BPT diagram). Triangles and circles represent the star-forming and AGN models, respectively. The gas-phase metallicity increases from left to right. The blue continuous line represents the separation line by Kauffmann et al. (2003), while the red dotted line represents the maximum starburst model by Kewley et al. (2001). The cyan and green dashed lines show the average location of star-forming galaxies at z = 0 and z = 2.3, respectively, from Kewley et al. (2013) and Shapley et al. (2015). The big red squares and dashed line highlight the AGN sequence by Richardson et al. (2014), with increasing ionization from bottom to top.