Abstract
We derive a new relation between the metallicity of Seyfert 2 Active Galactic Nuclei (AGNs) and the intensity of the narrow emission-lines ratio $N2O2$=log([N II]$\lambda$6584/[O II]$\lambda$3727). The calibration of this relation was performed determining the metallicity ($Z$) of a sample of 58 AGNs through a diagram containing the observational data and the results of a grid of photoionization models obtained with the Cloudy code. We find the new $Z/Z_\odot$-$N2O2$ relation using the obtained metallicity values and the corresponding observational emission line intensities for each object of the sample. Estimations derived through the use of this new calibration indicate that narrow line regions of Seyfert 2 galaxies exhibit a large range of metallicities ($0.3 \: < \: Z/Z_{\odot} \: < \:2.0$), with a median value $Z \approx Z_{\odot}$. Regarding the possible existence of correlations between the luminosity $L(\rm H\beta$), the electron density, and the color excess E(B$-$V) with the metallicity in this kind of objects, we do not find correlations between them.
Highlights
Active galactic nuclei (AGNs) present in their spectra strong emission lines of heavy elements, measured even for objects at high redshifts
Dors et al (2015) found that the Te-method does not work for AGNs. These authors examined the relation between oxygen abundances in the narrow-line regions (NLRs) of AGNs estimated from the Te-method, the strong-line method and through central intersect abundances in the host galaxies determined from the radial abundance gradients
We proposed here a metallicity indicator for Seyfert 2 (Sy2) AGNs based on the narrow emission-line intensity ratio N2O2 = log([N II] λ6584/[O II]
Summary
Active galactic nuclei (AGNs) present in their spectra strong emission lines of heavy elements, measured even for objects at high redshifts. These authors examined the relation between oxygen abundances (generally used as metallicity tracer) in the narrow-line regions (NLRs) of AGNs estimated from the Te-method, the strong-line method and through central intersect abundances in the host galaxies determined from the radial abundance gradients They found that the Te-method underestimates the expected oxygen abundances by 0.8 dex and that this fact could be due to the presence of a secondary heating (ionizing) source in addition to the radiation produced in the inner parts of the AGN [see Zhang, Liang & Hammer (2013), Prieto, Marco & Gallimore (2005), and Contini (2012). The strong-line method seems to be more reliable to be used in AGN metallicity determinations
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