Evidence of extreme ionization conditions and low metallicity in GHZ2/GLASS-z12 from a combined analysis of NIRSpec and MIRI observations

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GHZ2/GLASS-z12 has been recently observed by JWST with both NIRSpec and MIRI spectrographs, making it the most distant galaxy (zspec=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 Hα), 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 (ΣSFR ~ 10²-10³ Msun/yr/kpc²), with a high ionization parameter (log₁₀(U) =-1.75 ± 0.16), a high ionizing photon production efficiency log(ξᵢₒₙ) = 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.