Evidence of extreme Ionization conditions and low metallicity in GHZ2/GLASS-Z12 from a combined analysis of NIRSpec and MIRI observations

GHZ2/GLASS-z12, one of the most distant galaxies found in JWST observations, has been recently observed with both the NIRSpec and MIRI spectrographs, establishing a spectroscopic redshift z spec = 12.34 and making it the first system at z > 10 with complete spectroscopic coverage from rest-frame UV to optical wavelengths. This galaxy is identified as a strong C IV λ1549 emitter (EW = 46 Å) with many other detected emission lines, such as N IV] λ1488, He II λ1640, O III] λ λ1661,1666, N III] λ1750, C III] λ λ1907,1909, [O II] λ λ3726,3729, [Ne III] λ3869, [O III] λ λ4959,5007, and Hα, including a remarkable detection of the O III Bowen fluorescence line at rest frame λ = 3133 Å. We analyze in this paper the joint NIRSpec + MIRI spectral data set. Combining six optical strong-line diagnostics (namely R2, R3, R23, O32, Ne3O2, and Ne3O2Hd), we find extreme-ionization conditions, with log10 ([O III] λ λ4959,5007/[O II] λ λ3726,3729) = 1.39 ± 0.19 and log10 ([Ne III] λ3869/[O II] λ λ3726,3729) = 0.37 ± 0.18 in stark excess compared to typical values in the interstellar medium (ISM) at lower redshifts. These line properties are compatible either with an active galactic nucleus (AGN) or with a compact, very dense star-forming environment (ΣSFR ≃ 10^2–10^3 M ⊙ yr‑1 kpc‑2 and ΣM* ≃ 10^4–10^5 M ⊙ pc‑2), with a high ionization parameter (log10(U) =‑1.75 ± 0.16), a high ionizing photon production efficiency log(ξion)=25.7‑0.1+0.3 , and a low gas-phase metallicity (also confirmed by the direct, T e method) ranging between 4% and 11% Z ⊙, indicating a rapid chemical enrichment of the ISM in the past few megayears. These properties also suggest that a substantial amount of ionizing photons (∼10%) are leaking outside of GHZ2 and starting to reionize the surrounding intergalactic medium, possibly due to strong radiation-driven winds. 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; and (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.