GA-NIFS: An extremely nitrogen-loud and chemically stratified galaxy at z∼ 5.55
arxiv(2024)
摘要
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 (∼ 10^5 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]λ 6087 and potentially [FeXIV]λ 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 (∼ 10^3 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.
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