Evidence of Extreme Ionization Conditions and Low Metallicity in GHZ2/GLASS-Z12 from a Combined Analysis of NIRSpec and MIRI Observations
Antonello Calabrò, M. Castellano, Jorge A. Zavala, L. Pentericci, Pablo Arrabal Haro, Tom J. L. C. Bakx, D. Burgarella, Caitlin M. Casey, Mark Dickinson, Steven L. Finkelstein, A. Fontana, Mario Llerena, Sara Mascia, E. Merlin, Ikki Mitsuhashi, Lorenzo Napolitano, D. Paris, Pablo G. Pérez‐González, Guido Roberts-Borsani, P. Santini, Tommaso Treu, E. Vanzella
Abstract
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 > 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.