Evidence of Extreme Ionization Conditions and Low Metallicity in GHZ2/GLASS-Z12 from a Combined Analysis of NIRSpec and MIRI Observations

Abstract 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 &gt; 10 with complete spectroscopic coverage from rest-frame UV to optical wavelengths. This galaxy is identified as a strong C iv λ 1549 emitter (EW = 46 Å) with many other detected emission lines, such as N iv ] λ 1488 , He ii λ 1640 , O iii ] λ λ 1661,1666 , N iii ] λ 1750 , C iii ] λ λ 1907,1909 , [O ii ] λ λ 3726,3729 , [Ne iii ] λ 3869 , [O iii ] λ λ 4959,5007 , and H α , 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 log 10 ([O III ] λ λ 4959,5007 /[O II ] λ λ 3726,3729 ) = 1.39 ± 0.19 and log 10 ([Ne III ] λ 3869 /[O II ] λ λ 3726,3729 ) = 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 (Σ SFR ≃ 10 2 –10 3 M ⊙ yr −1 kpc −2 and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi mathvariant="normal">Σ</mml:mi> <mml:msub> <mml:mi>M</mml:mi> <mml:mo mathvariant="italic">*</mml:mo> </mml:msub> </mml:msub> </mml:math> ≃ 10 4 –10 5 M ⊙ pc −2 ), with a high ionization parameter (log 10 ( U ) =−1.75 ± 0.16), a high ionizing photon production efficiency <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mrow> <mml:mi>ξ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>ion</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>25.7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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.