On the central abundances of active galactic nuclei and star-forming galaxies

Abstract

We examine the relation between oxygen abundances in the narrow-line regions (NLRs) of active galactic nuclei (AGNs) estimated from the optical emission lines through the strong-line method (the theoretical calibration of Storchi-Bergmann et al. 1998), via the direct T e -method, and the central intersect abundances in the host galaxies determined from the radial abundance gradients. We found that the T e -method underestimates the oxygen abundances by up to ∼2 dex (with average value of ∼ 0.8 dex) compared to the abundances derived through the strong-line method. This confirms the existence of the so-called "temperature problem" in AGNs. We also found that the abundances in the centres of galaxies obtained from their spectra trough the strong-line method are close to or slightly lower than the central intersect abundances estimated from the radial abundance gradient both in AGNs and Star-forming galaxies. The oxygen abundance of the NLR is usually lower than the maximum attainable abundance in galaxies (∼2 times the solar value). This suggests that there is no extraordinary chemical enrichment of the NLRs of AGNs.

New metallicity calibration for Seyfert 2 galaxies based on the N 2 O 2 index

“…Photoionization model grids assuming these ranges of U and Z/Z ⊙ values describe emission line intensities observed practically at all wavelengths (Nagao et al 2002(Nagao et al , 2006Groves et al 2006;Dors et al 2012Dors et al , 2014Dors et al , 2015. Models assuming the presence of dust in the gas phase do not reproduce the majority of emission-line intensities of AGNs (Nagao et al 2006;Matsuoka et al 2009;Dors et al 2014), hence all the considered models in this work were dust free.…”

“…Observational data of 47 Sy2 compiled by Dors et al (2015) and 13 observed by Dopita et al (2015) were considered. From this sample, only the objects that meet the criteria proposed by Kewley et al (2001) to distinguish objects ionized by massive stars from those containing an active galactic nucleus (AGN) and/or gas shock were considered (see Fig.…”

“…The final sample consists of 58 objects: 46 compiled by Dors et al (2015) and 12 observed by Dopita et al (2015). In Figure 1 the objects of our final sample and a curve representing the criterion defined by (Kewley et al 2001) are shown.…”

“…In fact, Pilyugin (2003) showed that there is a good agreement (at least for the solar neighbourhood) between oxygen determinations based on the T e -method and the ones derived through observations of the weak interstellar O Iλ1356 line towards the stars (see also Moss et al 2002;Deharveng et al 2000;Rolleston et al 2000;Meyer et al 1998). However, Dors et al (2015) found that the T e -method does not work for AGNs. These authors examined the relation between oxygen abundances (generally used as metallicity tracer) in the narrow-line regions (NLRs) of AGNs estimated from the T e -method, strong-line method and through central intersect abundances in the host galaxies determined from the radial abundance gradients.…”

“…Therefore, the strong-line method seems to be more reliable to be used in AGN metallicity determinations. Dors et al 2015;Richardson et al 2014;Batra & Baldwin 2014;Du et al 2014;Wang et al 2011;Dhanda et al 2007;Baldwin et al 2003;Hamann et al 2002;Ferland et al 1996;Hamann & Ferland 1993, it seems that the unique calibrations available in the literature are the ones proposed by Storchi-Bergmann et al (1998) and by Dors et al (2014), who used photoionization model results to obtain expressions easily applicable considering optical and ultraviolet emission lines, respectively.…”