Key points are not available for this paper at this time.
GHZ2/GLASS-z12 has been recently observed by JWST with both NIRSpec and MIRI spectrographs, making it the most distant galaxy (zₒ₄₂=12. 34) with complete spectroscopic coverage from rest-frame UV to optical. It is identified as a strong CIV₁₅₄₉ emitter with many other detected emission lines (NIV], HeII, OIII], NIII], CIII], OII, NeIII, OIII, and H), including a remarkable OIII₁₃₃₃ 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 (ₒ₅ₑ 10²-10³ Msun/yr/kpc²), with a high ionization parameter (₁₀ (U) =-1. 75 0. 16), a high ionizing photon production efficiency (₈₎₍) = 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.
Calabrò et al. (Tue,) studied this question.