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 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 ($z \: {\lesssim}\: 0.4$) 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 $\rm km \: s^{-1}$ and imply a narrower metallicity range ($0.6 \: {\lesssim}\: (Z/Z_{\odot }) \: {\lesssim}\: 1.6$) than those derived using pure photoionization models. Our results indicate that shock velocity in AGNs can not be estimated using standard optical line ratio diagrams, based on integrated spectra. Our models predict a different temperature structure and $\rm O^{+}$/O and $\rm O^{2+}$/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/Hα) indexes which agree with that derived from pure photoionization models.

Diagnostic diagrams for the classification of galactic nuclei: comparison of traditional and alternative diagrams

“…One again, we note that the model for α = −1.4 lies to the region that is occupied by observational data of AGN, the models are consistent in their own distribution, with objects that have lower ionization parameters being more to the bottom, according to the LINER locus, and the cases of higher metallicity being more to the right in comparison to the models of the same color (same ionization parameter and spectral index), which is expected for the sum of the nitrogen and sulfur line ratios. We also see a trend with regard to the spectral index values, with flatter spectra located more to the lower right, but such behavior is not as explicit as in the case of The issue with modeling the [O i] line, however, is not something exceptional from this work, and has been faced in other works that relied on simplistic photoionization models (e.g., Veilleux and Osterbrock, 1987;Stasińska et al, 2006;Dopita et al, 2013;Dors et al, 2021). Some of the changes that can provide a stronger emission of the neutral oxygen optical emission line is by setting a harder ionization field (by setting higher temperatures, non-equilibrium heating or including shocks), higher densities, or other contributions from dust (Dopita, 1997;Kewley et al, 2006).…”

“…It should be noted that while in the star-forming sequence we have only H ii region-like objects, the right branch of the diagnostic diagrams includes also objects that are not photoionized by an AGN. The presence of shocks contributes to the high intensities of the emission line ratios, and the higher the shock velocity, the higher [O iii]/Hβ (Spence et al, 2016;Dors et al, 2021). The fastest shocks, however, are still linked to AGN activity, usually as a result of radio jets and/or outflows from the ISM.…”

“…With regard to models of AGNs, we have models that consider purely the photoion-ization from the central sources (e.g., Halpern and Steiner, 1983;Stasińska et al, 2006;Feltre et al, 2016), models that consider shocks (e.g., Evans and Dopita, 1985;Dopita and Sutherland, 1995;Dors et al, 2021), and other complex models, as the combination of matter-and radiation-bounded clouds (e.g., Binette et al, 1996), locally emitting clouds (e.g., Ferguson et al, 1997), and dusty models dominated by radiation pressure (e.g., Dopita et al, 2002;Groves et al, 2004). Such complex models to describe AGNs are explained in more detail in Appendix A.…”

“…Shocks might affect the intensity and shape of the emission lines, but they are usually not the main drivers of the ionization in AGNs (see Dors et al, 2021, for cases where shock dominates over photoionization), so photoionization from the central source has to be included in the models. It is interesting to consider both shocks and photoionization by a power-law simultaneously, as in Dors et al (2021); we go into further details about shock models on Appendix A.3. The techniques for modeling AGN ionizing radiation fields are still being developed, with examples as in Groves et al (2004) and Thomas et al (2016).…”

“…The cooling mechanisms in AGNs and star-forming nuclei are collisions, recombination, and free-free interactions dissipating the kinetic energy of the electron gas in the form of electromagnetic radiation (Flury and Moran, 2020). The elements considered in the standard diagnostic diagrams (oxygen, nitrogen, and sulphur) are the main contributors to the cooling provided by metals in nebular gas (Dors et al, 2021).…”