GA-NIFS: An extremely nitrogen-loud and chemically stratified galaxy at $z\sim 5.55$

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. Based on the rest-frame UV emission lines, which trace high-density ($\sim {10}^5{\mathrm{c}\mathrm{m}}^{-3}$) and highly ionized gas, we derived an abundance ratio of $\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{N}\mathrm{/}\mathrm{O})=0.42_{-0.10}^{+0.13}$. At an estimated metallicity of $0.2Z_{\odot }$, 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 $\mathrm{l}\mathrm{o}\mathrm{g}(\mathrm{C}\mathrm{/}\mathrm{O})=-0.38_{-0.11}^{+0.13}$, which is not significantly higher than those in local galaxies and stars with similar metallicities. We also found potential detection of coronal lines including [FeVII]$\lambda 6087$ and [FeXIV]$\lambda 5303$, both blended with [CaV]. We inferred a range of Fe abundances compatible with those in local stars and galaxies. Overall, the chemical abundance pattern of GS_3073 is compatible with enrichment by super-massive stars with $M_{\ast }\gtrsim 1000M_{\odot }$, asymptotic giant branch (AGB) stars, or Wolf-Rayet (WR) stars. Interestingly, when using optical emission lines which trace lower density ($\sim {10}^3{\mathrm{c}\mathrm{m}}^{-3}$) 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 galaxies, while the bulk of the galaxies evolves more normally.