Multiline Assessment of Narrow-line Regions in z ∼3 Radio Galaxies

In this paper, we use high-quality rest-UV spectra of three radio galaxies at z ∼3 observed with the FORS2 camera on the Very Large Telescope to measure the flux of several emission lines, including relatively faint ones, such as N iv]λ1486, O iii]λ1663, and [Ne iv]λ2424. Additionally, we collect fluxes of faint rest-UV emission lines in 12 z ∼3 radio galaxies from the literature. Previously, physical and chemical properties of narrow-line regions (NLRs) in high-z active galactic nuclei (AGNs) have been investigated mostly by using only strong rest-UV emission lines (e.g., N v λ1240, C iv λ1549, He ii λ1640, and C iii]λ1909). Such strong-line diagnostics are based on various assumptions due to the limitation in the number of available emission-line constraints. In this work, both physical and chemical properties of NLR clouds in each object are estimated by fitting detailed photoionization models to the measured emission-line fluxes. We confirm that the metallicity of NLRs in AGNs at z ∼3 is solar or supersolar, without assumptions about the gas density and ionization parameter thanks to the constraints from the faint emission lines. This result suggests that high-z radio galaxies are already chemically mature at z ∼3.