Metallicity evolution of AGNs from UV emission lines based on a new index

Abstract

We analyzed the evolution of the metallicity of the gas with the redshift for a sample of AGNs in a very wide redshift range (0 < z < 4) using ultraviolet emission-lines from the narrow-line regions (NLRs) and photoionization models. The new index C43=log[(C iv+C iii])/He ii] is suggested as a metallicity indicator for AGNs. Based on this indicator, we confirmed the no metallicity evolution of NLRs with the redshift pointed out by previous works. We found that metallicity of AGNs shows similar evolution than the one predicted by cosmic semi-analytic models of galaxy formation set within the Cold Dark Matter merging hierarchy (for z 3). Our results predict a mean metallicity for local objects in agreement with the solar value (12+log(O/H)=8.69).This value is about the same that the maximum oxygen abundance value derived for the central parts of local spiral galaxies. Very low metallicity log(Z/Z ⊙ ) ≈ −0.8 for some objects in the range 1.5 < z < 3 is derived.

Ultraviolet/Optical Emission of the Ionized Gas in AGN: Diagnostics of the Ionizing Source and Gas Properties

“…Figure 1 shows the predictions from photoionization models of both AGN and star-forming galaxies, for different values of metallicity and ionization parameter in two diagnostic diagrams, namely C III]λ1909/He II λ1640 vs. C IV λ1550/He II λ1640 (left panel) and C IV λ1550/C III]λ1909 vs. C IV λ1550/He II λ1640 (right panel). Note that model predictions agree well with data measurements of AGN (Dors et al, 2014) and star-forming galaxies (Stark et al, 2014) currently available in the literature.…”

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

“…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.…”

New quantitative nitrogen abundance estimations in a sample of Seyfert 2 active galactic nuclei

“…Therefore, AGN metallicity determinations play a fundamental role in the knowledge of the chemical evolution of galactic nuclei (e.g. Dors et al 2014;Matsuoka et al 2009;Nagao et al 2006). They also provide an indirect understanding of the star formation history for the central parts of galaxies (Hamann & Ferland 1992, 1993Collin & Zahn 1999;Wang et al 2011).…”

“…However, most of these works have considered only the metallicity parametrized through the oxygen abundance determinations (e.g. Dors et al 2015Dors et al , 2014Richardson et al 2014;Matsuoka et al 2009;Nagao et al 2006;Groves et al 2006;Storchi-Bergmann et al 1998;Cruz-Gonzalez et al 1991).…”

“…These authors found that models assuming nitrogen abundances ranging from 0.5 to 3 times the solar values reproduce the observational data (see also Yu & Hwang 2011;Bradley, Kaiser & Baan 2004). Moreover, the majority of the photoionization models built in order to reproduce AGN emission line intensities assumed a N/O-O/H relation derived from H ii region abundance determinations (Dors et al 2015(Dors et al , 2014Matsuoka et al 2009;Groves et al 2006;Nagao et al 2006). If these relations are not valid for AGN, incorrect metallicity determinations or calibrations between metallicity and strong emission-lines could be obtained (Pérez-Montero & Contini 2009).…”