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We analyze the validity of optical diagnostic diagrams relying onemission-lines ratios and in the context of classifying Active Galactic Nuclei(AGNs) according to the cosmic metallicity evolution in the redshift range 0 <z < 11.2. In this regard, we fit the results of chemical evolution models(CEMs) to the radial gradients of the N/O abundances ratio derived throughdirect estimates of electron temperatures (Te-method) in a sample of four localspiral galaxies. This approach allows us to select representative CEMs andextrapolate the radial gradients to the nuclear regions of the galaxies in oursample, inferring in this way the central N/O and O/H abundances. The nuclearabundance predictions for theoretical galaxies from the selected CEMs, atdistinct evolutionary stages, are used as input parameters in AGNphotoionization models built with the Cloudy code. We found that standard BPTdiagnostic diagrams are able to classify AGNs with oxygen abundances 12+logO/H> 8.0 [(Z/Zsolar) > 0.2) preferably found at redshift z > 4. On the other hand,the HeII4685/Hbeta versus [N II]6584/Halpha diagram produces a reliable AGNclassification independent of the evolutionary stage of these objects.

Cosmic metallicity evolution of Active Galactic Nuclei: Implications for optical diagnostic diagrams