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

Abstract

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 ] λ 1486, O iii ] λ 1663, and [Ne iv ] λ 2424. 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 λ 1240, C iv λ 1549, He ii λ 1640, and C iii ] λ 1909). 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.