Date: 11/11/2024
Harvard ADS: GA-NIFS: an extremely nitrogen-loud and chemically stratified galaxy at z 5.55
Paper abstract: We report the chemical abundance pattern of GS_3073, 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 (~ 10^5~{\rm cm^{-3}}) and highly ionized gas, we derive \rm \log (N/O) = 0.42^{+0.13}_{-0.10}. At an estimated metallicity of 0.2~Z_{\odot }, this is the most extreme nitrogen-rich object found by JWST thus far. In comparison, the relative carbon abundance derived is \rm \log (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]\lambda 6087 and [Fe XIV]\lambda 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 GS_3073 is compatible with enrichment by supermassive stars with M_* \gtrsim 1000~M_\odot, asymptotic giant branch stars, or Wolf-Rayet stars. Interestingly, when using optical emission lines that trace lower density (~ 10^3~{\rm 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.
