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\na sample of AGNs in a very wide redshift range (0<z<4) using ultraviolet\nemission-lines from the narrow-line regions (NLRs) and photoionization models.\nThe new index C43=log(CIV+CIII])/HeII is suggested as a metallicity indicator\nfor AGNs. Based on this indicator, we confirmed the no metallicity evolution of\nNLRs with the redshift pointed out by previous works. We found that metallicity\nof AGNs shows similar evolution than the one predicted by cosmic semi-analytic\nmodels of galaxy formation set within the Cold Dark Matter merging hierarchy\n(for z < 3). Our results predict a mean metallicity for local objects in\nagreement with the solar value (12+log(O/H)=8.69). This value is about the same\nthat the maximum oxygen abundance value derived for the central parts of local\nspiral galaxies. Very low metallicity log(Z/Z_{\\odot})~ -0.8 for some objects\nin the range 1.5 < z <3 is derived.\n