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

Abstract

We analysed 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 semianalytic models of galaxy formation set within the cold dark matter merging hierarchy (for z ≲ 3).Our results predict amean metallicity for local objects in agreement with the solar value (12+log(O/H)=8.69). This value is about the same that themaximum 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.