Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Xihan Ji, Hannah Übler, Roberto Maiolino, Francesco D’Eugenio, Santiago Arribas
| Summary:
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 ($sim 10^5~{rm cm}^{-3}$) and highly
ionized gas, we derived an abundance ratio of $rm log(N/O) =
0.42^{+0.13}_{-0.10}$. 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 from the rest-frame UV emission lines 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 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 $M_* gtrsim 1000~M_odot$, asymptotic giant branch
(AGB) stars, or Wolf-Rayet (WR) stars. Interestingly, when using optical
emission lines which trace lower density ($sim 10^3~{rm cm}^{-3}$) 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 galaxies, while the bulk of
the galaxies evolves more normally.
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