Chemical abundances of Seyfert 2 AGNs, II: N2 metallicity calibration based on SDSS

Abstract

We present a semi-empirical calibration between the metallicity (Z) of Seyfert 2 active galactic nuclei and the N2 = log([N II]λ6584/H a) emission-line intensity ratio. This calibration was derived through the [OIII]λ5007/[O II]λ3727 versus N2 diagram containing observational data and photoionization model results obtained with the CLOUDY code. The observational sample consists of 463 confirmed Seyfert 2 nuclei (redshift z ≤ 0.4) taken from the Sloan Digital Sky Survey DR7 data set. The obtained Z-N2 relation is valid for the range 0.3 ≤ (Z/Z⊙) ≤ 2.0 that corresponds to -0.7 ≤ (N2) ≤ 0.6. The effects of varying the ionization parameter (U), electron density and the slope of the spectral energy distribution on the Z estimations are of the order of the uncertainty produced by the error measurements of N2. This result indicates the large reliability of our Z -N2 calibration. A relation between U and the [OIII]/[O II] line ratio, almost independent of other nebular parameter, was obtained.