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

TL;DR

This paper presents ultra-deep rest-frame optical spectroscopy of the $z=14.18$ galaxy JADES-GS-z14-0 using JWST/MIRI/LRS, detecting bright nebular lines ([OIII] and H$\alpha$) and constraining its dust, SFR, ionizing photon production, and chemical abundances. Through standard strong-line diagnostics and detailed photoionization modeling with the Cue emulator, the authors derive complementary metallicity estimates: $[O/H]\approx -1.05$ dex from strong-line methods (≈10% of solar) and $[O/H]\approx -0.28$ dex (≈50% solar) from modeling, with a high ionization parameter $\log U$ around $-1.5$ and elevated electron density $n_H$ of several hundred cm$^{-3}$. They find an SFR of $\approx 10\ M_{\odot}\,\mathrm{yr^{-1}}$ and a high SFR surface density $\Sigma_{\mathrm{SFR}}\approx 23\ M_{\odot}\,\mathrm{yr^{-1}\,kpc^{-2}}$, along with an ionizing photon production efficiency $\log_{10}(\xi_{\mathrm{ion}})\approx 25.3$ Hz erg$^{-1}$. These properties place JADES-GS-z14-0 among the most active $z>10$ galaxies, with implications for rapid metal enrichment and reionization, and highlight a significant tension with current galaxy formation simulations, which would require higher star-formation efficiencies or bursty histories to reproduce such systems. The work underscores the transformative role of deep MIRI observations for constraining the physical conditions in the earliest galaxies and guiding improvements in theoretical models of the break to cosmic dawn.

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. 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 $[\mathrm{OIII}] λλ4959,5007$ at $\approx 11 σ$ and $\mathrm{H}α$ at $\approx 4 σ$ 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 $Σ_{\mathrm{SFR}} \approx 23 \pm 5\ M_{\odot}/\mathrm{yr}/\mathrm{kpc}^{2}$, and ionizing photon production efficiency of $ξ_{\mathrm{ion}} \approx 10^{25.3 \pm 0.1}\ \mathrm{Hz/erg}$. Using standard strong-line diagnostics, we infer a gas-phase oxygen abundance of $[\mathrm{O/H}] \approx -1.1 \pm 0.4$ ($\approx 10\%\ Z_{\odot}$), 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 $[\mathrm{O/H}] \approx -0.3_{-0.4}^{+0.4}$ ($\approx 50\%\ Z_{\odot}$) and $\mathrm{log}_{10}(U) \approx -1.5_{-0.4}^{+0.3}$. 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. Existing simulations are unable to reproduce the empirical and inferred properties of JADES-GS-z14-0. This work demonstrates an important step toward understanding the formation of the first stars and heavy elements in the Universe. [Abridged]

Figures (8)

• Figure 1: Slit locations for three separate visits. Our target, JADES-GS-z14-0, was observed using the MIRI/LRS in slit mode across three separate visits with two dithers along the slit nod for each visit. We show the six distinct slit locations alongside the JWST/NIRCam imaging (using F444W-F277W-F115W filters as an RGB false-color mosaic). MIRI/LRS's slit size is $4.7\ \mathrm{arcsec}$ in length and $0.51\ \mathrm{arcsec}$ in width. Two-thirds of the observations were successful in placing JADES-GS-z14-0 within the slit (observations $\#2$ and $\#3$); the remaining one-third of the observations were not successful because of failed target acquisition caused by an anomalous cosmic-ray event (observation $\#1$).
• Figure 2: The rest-frame optical spectrum of JADES-GS-z14-0.Top panel: The final MIRI/LRS 2D spectrum is provided. The corresponding color bar for the measured signal-to-noise ratio per pixel is also shown. JWST/MIRI's detector plate scale is $0.11\ \mathrm{arcsec/pixel}$, as shown by the $y$-axis label. Blue, horizontal lines roughly indicate the wavelength-dependent profiles used for the 1D optimal extraction while the blue, vertical lines indicate the locations for some of the strongest rest-frame optical emission lines. Bottom panel: The final MIRI/LRS 1D spectrum is provided. There are two spectroscopic features that are clearly detected above the noise level ($> 3 \sigma$) in both the 2D and 1D spectra, in addition to the continuum, which is marginally detected ($> 1 \sigma$ per wavelength bin at $\lambda_{\mathrm{obs}} \lesssim 8\ \mu\mathrm{m}$). The detection of these rest-frame optical emission lines and their scientific interpretation are the focus of this work, while the rest-frame optical continuum will be discussed and interpreted in a forthcoming manuscript (Helton et al., in preparation). The grey, vertical lines indicate the locations of the Balmer continuum limit for JADES-GS-z14-0 and the $\mathrm{Pa}\alpha$ line for the neighboring foreground galaxy at $z = 3.475$ (see also Figure \ref{['fig:Slit_Locations']}).
• Figure 3: Zoom-in views around the strongest rest-frame optical emission lines. From left to right, we show the final MIRI/LRS 1D spectrum centered around $[\mathrm{OII}] \lambda\lambda 3727{,}3729$, $\mathrm{H}\beta + [\mathrm{OIII}] \lambda\lambda 4959{,}5007$, $\mathrm{HeI} \lambda 5876$, and $\mathrm{H}\alpha$, respectively. Top panels: The best-fit continua and line profiles are shown in red along with the measured flux densities shown by the black lines and the flux uncertainties ($1\sigma$) shown by the grey shaded regions. Blue, vertical lines once again indicate the locations for some of the strongest emission lines. Bottom panels: The residuals of the best-fit models compared with the observations are shown by solid black lines and grey shaded regions. We report the reduced chi-squared statistics to demonstrate the quality of our fits.
• Figure 4: $\mathrm{H}\alpha$ line luminosity versus absolute UV magnitude.Left panel: These measurements are shown for a few different samples of high-redshift star-forming galaxies at $z > 3$. These include $N \approx 1500$ galaxies at $z > 3$ from the JADES DR4, represented by the grey circles, and $N \approx 850$ of those same galaxies with $\mathrm{H}\beta$ well-detected at $> 4 \sigma$, represented by the green squares Curtis-Lake:2025Scholtz:2025b. For galaxies at $z \gtrsim 6$ in the JADES DR4, $\mathrm{H}\alpha$ has shifted beyond JWST/NIRSpec's wavelength coverage, so we use the $\mathrm{H}\beta$ line luminosities instead and assume zero dust attenuation, consistent with Case B recombination. Three galaxies at $z > 10$ from the literature with previous MIRI/Spectroscopy are illustrated by the purple diamonds. These include, in order of decreasing brightness, GN-z11 at $z = 10.60$Bunker:2023Alvarez-Marquez:2025, GHZ2/GLASSz12 at $z = 12.34$Castellano:2024Zavala:2025, and MACS0647-JD1 at $z = 10.17$Hsiao:2024aHsiao:2024b. Our measurements for JADES-GS-z14-0 at $z = 14.18$ are depicted by the blue star and represent the most distant detection of $\mathrm{H}\alpha$. As a point of comparison, we provide brown lines to show the relation between $L_{\mathrm{H}\alpha}$ and $M_{\mathrm{UV}}$ as parametrized by the ionizing photon production efficiency. Right panel: Histograms showing the distributions of $\mathrm{H}\alpha$ line luminosities, which are used as a proxy for the recent star-formation rate. Solid lines represent measurements for galaxies at $z > 10$ while grey dashed and dotted lines represent medians and $68\%$ confidence intervals for the samples of galaxies at $z > 3$ from JADES DR4. JADES-GS-z14-0 is among the most actively star-forming galaxies at $z > 3$ with spectroscopic confirmation.
• Figure 5: Ionizing photon production efficiency versus both spectroscopic redshift and absolute UV magnitude.Left panel: We show the same three samples of star-forming galaxies at $z > 3$ from Figure \ref{['fig:LHalpha_vs_MUV']} using a consistent plotting scheme. For comparison, the canonical values that have been predicted and observed in galaxies at $z \lesssim 4$ are shown by the grey shaded region Madau:1999Bouwens:2016 while the approximate stellar population maximum is shown by the black line Schaerer:2025. Among the four galaxies at $z > 10$ with these measurements, there are three that are similarly efficient at producing ionizing photons and include JADES-GS-z14-0, GHZ2/GLASSz12, and GN-z11. No dust corrections have been applied to these galaxies. The observed redshift evolution in the lower envelope of green points is caused by observational limitations and burstier SFHs at high redshifts, while the evolution in the upper envelope is caused by smaller dust reservoirs at high redshifts. Right panel: The faintest galaxies are observed as the most efficient producers of ionizing photons.