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

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 ($t_{\mathrm{obs}} \approx 34\ \mathrm{hr}$ ). This spectrum covers the brightest rest-frame optical nebular emission lines for the luminous galaxy JADES-GS-z14-0 at $z > 14$ . Most notably, we detect $[\mathrm{OIII}] \lambda\lambda 4959{,}5007$ at $\approx 11 \sigma$ and $\mathrm{H}\alpha$ at $\approx 4 \sigma$ with these ultra-deep observations. These lines reveal that JADES-GS-z14-0 has low dust attenuation with a recent star-formation rate of $\mathrm{SFR} \approx 10 \pm 2\ M_{\odot} / \mathrm{yr}$ , star-formation rate surface density of $\Sigma_{\mathrm{SFR}} \approx 23 \pm 5\ M_{\odot}/\mathrm{yr}/\mathrm{kpc}^{2}$ , and ionizing photon production efficiency of $\xi_{\mathrm{ion}} \approx 10^{25.3 \pm 0.1}\ \mathrm{Hz/erg}$ . Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of $\mathrm{log}_{10}(\mathrm{O/H}) + 12 \approx 7.6 \pm 0.4$ ($\approx 10\%\ Z_{\odot}$ ), carbon-to-oxygen ratio of $[\mathrm{C/O}] \approx -0.4 \pm 0.4$ , ionization parameter of $\mathrm{log}_{10}(U) \gtrsim -2.4$ , and density of $n_{\mathrm{H}} \approx 720 \pm 210\ \mathrm{cm}^{-3}$ . Using detailed photoionization modeling, we instead derive $\mathrm{log}_{10}(\mathrm{O/H}) + 12 \approx 8.4_{-0.4}^{+0.4}$ ($\approx 50\%\ Z_{\odot}$ ), $\mathrm{log}_{10}(U) \approx -1.5_{-0.4}^{+0.3}$ , and $n_{\mathrm{H}} \approx 520_{-310}^{+480}\ \mathrm{cm}^{-3}$ . The inferred properties of JADES-GS-z14-0 are similar to those measured for similarly luminous galaxies at $z > 10$ with previous MIRI/Spectroscopy, such as GHZ2/GLASSz12, GN-z11, and MACS0647-JD1. These results suggest extreme ionization conditions and rapid metal enrichment less than $300$ million years after the Big Bang. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. The spectrum that we obtained with the MIRI/LRS includes a tentative detection of the rarely seen $\mathrm{HeI} \lambda 5876$ line, indicating possible contributions from shocked nebular gas. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe.

eprint arXiv:2512.19695