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

Understand

We analyze the validity of optical diagnostic diagrams relying on emission-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 through direct estimates of electron temperatures (Te-method) in a sample of four local spiral galaxies.
• This approach allows us to select representative CEMs and extrapolate the radial gradients to the nuclear regions of the galaxies in our sample, inferring in this way the central N/O and O/H abundances.
• The nuclear abundance predictions for theoretical galaxies from the selected CEMs, at distinct evolutionary stages, are used as input parameters in AGN photoionization models built with the Cloudy code.