Evidence of extreme ionization conditions and low metallicity in GHZ2/GLASS-z12 from a combined analysis of NIRSpec and MIRI observations
arxiv(2024)
摘要
GHZ2/GLASS-z12 has been recently observed by JWST with both NIRSpec and MIRI
spectrographs, making it the most distant galaxy (z_spec=12.34) with
complete spectroscopic coverage from rest-frame UV to optical. It is identified
as a strong CIV_1549 emitter with many other detected emission lines (NIV],
HeII, OIII], NIII], CIII], [OII], [NeIII], [OIII], and Hα), including a
remarkable OIII_1333 Bowen fluorescence line. We analyze in this paper the
joint NIRSpec+MIRI spectral data set. Combining six optical diagnostics (namely
R2, R3, R23, O32, Ne3O2, and Ne3O2Hd), we find extreme ionization conditions,
with O32 =1.39 ± 0.19 and Ne3O2 =0.37 ± 0.18 in stark excess compared
to typical values in the ISM at lower redshifts. These line properties are
compatible either with an AGN or with a compact, very dense star-forming
environment (Σ_ SFR ∼ 10^2-10^3 Msun/yr/kpc^2), with a
high ionization parameter (log_10(U) =-1.75 ± 0.16), a high ionizing
photon production efficiency log(ξ_ ion) = 25.7_-0.1^+0.2, and a
low, although not pristine, metal content ranging between 5% and 11%
Z_⊙, indicating a rapid enrichment of the ISM in the last few Myrs. These
properties also suggest that a substantial amount of ionizing photons (∼
10%) are leaking outside. The general lessons learned from GHZ2 are the
following: (i) the UV to optical combined nebular indicators are broadly in
agreement with UV-only or optical-only indicators. (ii) UV+optical diagnostics
fail to discriminate between an AGN and star-formation in a low metallicity,
high density, and extreme ionization environment. (iii) comparing the nebular
line ratios with local analogs may be approaching its limits at z ≳ 10,
as this approach is potentially challenged by the unique conditions of star
formation experienced by galaxies at these extreme redshifts.
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