Multi-line assessment of narrow-line regions in $z \sim$ 3 radio galaxies

TL;DR

This study uses detailed photoionization modeling of UV emission lines in high-redshift radio galaxies to determine their narrow-line region properties, revealing they are already chemically mature at z~3.

Contribution

First detailed photoionization analysis of multiple faint UV emission lines in z~3 radio galaxies, providing robust metallicity estimates without strong-line assumptions.

Findings

NLR metallicity in z~3 radio galaxies is solar or super-solar.

High-z radio galaxies are chemically mature by z~3.

Faint emission lines help constrain physical conditions.

Abstract

In this paper, we utilize high-quality rest-UV spectra of three radio galaxies at $z \sim$ 3 observed with VLT/FORS2 to measure the flux of several emission lines including relatively faint ones, such as NIV]$\lambda$1486, OIII]$\lambda$1663, and [NeIV]$\lambda$2424. Additionally, we collect fluxes of faint rest-UV emission lines in 12 $z \sim$ 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., NV$\lambda$1240, CIV$\lambda$1549, HeII$\lambda$1640, and CIII]$\lambda$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 $z \sim$ 3 is solar or super-solar, without assuming the gas density and ionization parameter thanks to the constrains from the faint emission lines. This result suggests that high-$z$ radio galaxies are already chemically matured at $z \sim$ 3.