GA-NIFS: An extremely nitrogen-loud and chemically stratified galaxy at z 5. 55

We report the chemical abundance pattern of GS\₃073, a galaxy at z=5. 55 which was previously confirmed to host an overmassive active black hole, by leveraging the detection of about 40 emission lines, combining JWST/NIRSpec observations and ground-based (VLT/VIMOS) data. By using rest-frame UV emission lines, which trace high-density (10⁵~ cm^-3) and highly ionized gas, we derived an abundance ratio of log (N/O) = 0. 46^+0. 12-₀. ₀₉. At an estimated metallicity of 0. 2~Z_, this is the most extreme nitrogen-rich object found by JWST thus far. In comparison, the relative carbon abundance derived from the rest-frame UV emission lines is log (C/O) = -0. 30^+0. 12-₀. ₀₉, which is not significantly higher than those in local galaxies and stars with similar metallicities. We also detected coronal lines, including FeVII 6087 and potentially FeXIV 5303. We inferred a high Fe abundance of Fe/O 0. 1. Overall, the chemical abundance pattern of GS\₃073 is compatible with enrichment by super-massive stars with M_* 1000~M_, ejecta from asymptotic giant branch (AGB) stars, or winds from Wolf-Rayet (WR) stars, although the WR scenario is less likely. Interestingly, when using optical emission lines which trace lower density (10³~ cm^-3) and lower ionization gas, we found a sub-solar N/O ratio. We interpret the difference in N/O derived from UV lines and optical lines as evidence for a stratified system, where the inner and denser region is both more chemically enriched and more ionized. Taking this luminous, well-studied system as a benchmark, our results suggest that nitrogen loudness in high-z galaxies is confined to the central, dense, and highly ionized region of the galaxy, while the bulk of the galaxy evolves more normally.