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ABSTRACT We report the chemical abundance pattern of GS₃073, a galaxy hosting an overmassive active black hole at z=5. 55, by leveraging observations from JWST/NIRSpec and Very Large Telescope/VIsible Multi-Object Spectrograph. Based on the rest-frame ultraviolet (UV) emission lines, which trace high-density (10⁵~ cm^{-3}) and highly ionized gas, we derive (N/O) = 0. 42^+0. 13-₀. ₁₀. 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 is (C/O) = -0. 38^+0. 13-₀. ₁₁, which is not significantly higher than those in local galaxies and stars with similar metallicities. We also found potential detection of Fe vii 6087 and Fe xiv 5303, both blended with Ca v. We inferred a range of Fe abundances compatible with those in local stars and galaxies. Overall, the chemical abundance pattern of GS₃073 is compatible with enrichment by supermassive stars with M_* 1000~M_, asymptotic giant branch stars, or Wolf–Rayet stars. Interestingly, when using optical emission lines that trace lower density (10³~ cm^-3) and lower ionization gas, we found a sub-solar N/O ratio, consistent with local galaxies at the same metallicity. 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. Our results suggest that nitrogen loudness in high-z galaxies might be confined to the central, dense, and highly ionized regions of the galaxies, while the bulk of the galaxies evolves more normally.
Ji et al. (Wed,) studied this question.