Ionizing Photon Production Efficiencies and Chemical Abundances at Cosmic Dawn Revealed by Ultradeep Rest-frame Optical Spectroscopy of JADES-GS-z14-0

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Abstract JWST has discovered an early period of galaxy formation that was more vigorous than expected, which has challenged our understanding of the early Universe. In this work, we present the longest spectroscopic integration ever acquired by JWST/MIRI ( t obs ≈ 51 hr). This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at z = 14.18. Most notably, we detect [O III ] λλ 4959, 5007 at ≈14 σ and H α at ≈4 σ with these ultradeep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star formation rate of SFR ≈ 8 ± 2 M ⊙ yr −1 , star formation rate surface density of Σ SFR ≈ 20 ± 5 M ⊙ yr −1 kpc −2 , and ionizing photon production efficiency of ξ ion ≈ 10 25.3±0.1 Hz erg −1 . Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of log 10 ( O / H ) + 12 ≈ 7.5 ± 0.2 (≈6% Z ⊙ ), carbon-to-oxygen ratio of [C/O] ≈ −0.4 ± 0.2, ionization parameter of log 10 ( U ) ≳ − 2.4 , and density of n H ≈ 690 ± 200 cm −3 . Using detailed photoionization modeling, we instead derive log 10 ( O / H ) + 12 ≈ 8 . 5 − 0.4 + 0.4 (≈60% Z ⊙ ), log 10 ( U ) ≈ − 1 . 4 − 0.4 + 0.3 , and n H ≈ 54 0 − 320 + 520 cm − 3 . The inferred properties of JADES-GS-z14-0 are similar

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