Chemical abundances of Seyfert 2 AGNs – IV. Composite models calculated by photoionization + shocks

Abstract

We build detailed composite models of photoionization and shock ionization based on the SUMA code to reproduce emission lines emitted from the Narrow Line Regions (NLR) of Seyfert 2 nuclei. The aim of this work is to investigate diagram active galactic nucleus (AGN) positions according to shock parameters, shock effects on the gas temperature and ionization structures and derive a semi-empirical abundance calibration based on emission-line ratios little sensitive to the shock presence. The models were used to reproduce optical (3000 < λ(Å) < 7000) emission line intensities of 244 local (⁠z≲0.4⁠) Seyfert 2s, whose observational data were selected from Sloan Digital Sky Survey DR7. Our models suggest that shocks in Seyfert 2 nuclei have velocities in the range of 50–300 kms−1 and imply a narrower metallicity range (⁠0.6≲(Z/Z⊙)≲1.6⁠) than those derived using pure photoionization models. Our results indicate that shock velocity in AGNs cannot be estimated using standard optical line ratio diagrams, based on integrated spectra. Our models predict a different temperature structure and O+/O and O2+/O fractional abundances throughout the NLR clouds than those derived from pure photoionization models, mainly in shock-dominated objects. This suggests that, in order to minimize the shock effects, the combination of emission-lines emitted by ions with similar intermediate ionization potential could be good metallicity indicators. Finally, we derive two calibrations between the N/O abundance ratio and the N2O2 = log([N II]λ6584/[O II]λ3727) and N2 = log([N II]λ6584/H α) indexes which agree with that derived from pure photoionization models.