Interpreting the ionization sequence in AGN emission-line spectra

We investigate the physical cause of the great range in the ionization level seen in the spectra of narrow-lined active galactic nuclei (AGN). We used a recently developed technique called mean field independent component analysis to identify examples of individual Sloan Digital Sky Survey galaxies whose spectra are not dominated by emission due to star formation (SF), which we therefore designate as AGN. We assembled high signal-to-noise ratio (S/N) composite spectra of a sequence of these AGN defined by the ionization level of their narrow-line regions (NLR), and extending down to very low ionization cases. We then used a local optimally emitting cloud (LOC) model to fit emission-line ratios in this AGN sequence, including the weak lines that can be measured only in the co-added spectra. These weak line ratios provide consistency checks on the density, temperature, abundances and ionizing continuum of Seyfert galaxies determined from strong-line ratios. After integrating over a wide range of clouds at different radii and densities, our models indicate that the radial extent of the NLR is the major parameter in determining the position of higher to moderate ionization AGN along our sequence. This provides a physical interpretation for their systematic variation. Higher ionization AGN contain optimally emitting clouds that are more concentrated towards the central continuum source than in lower ionization AGN. Our LOC models indicate that for the special set of objects that lie on our AGN sequence, the ionizing luminosity is anti-correlated with the NLR ionization level, and hence anticorrelated with the radial concentration and actual physical extent of the NLR. A possible interpretation that deserves further exploration is that the ionization sequence might be an age sequence where low ionization objects are older and have systematically cleared out their central regions by radiation pressure. We consider the alternative that our AGN sequence instead represents a mixing curve combining SF and AGN spectra in different proportions, but argue that while many galaxies in fact do have this type of composite spectra, our AGN sequence appears to be a special set of objects with negligible SF excitation.