GLIMPSE-DDT spectroscopic properties of faint-end galaxies at $z\sim6$: Towards first metal enrichment, dust production, and ionizing photon production

TL;DR

This paper probes the faint end of high-redshift galaxy populations ($z\sim6$) to understand early metal enrichment, dust production, and ionizing photon output. Utilizing deep JWST NIRCam imaging and NIRSpec spectroscopy of a strongly lensed field (AS1063) from the GLIMPSE survey, the authors assemble 16 UV-faint galaxies ($M_{ m UV}>-17$) at $5.5<z<6.5$ and derive metallicities, dust content, and $\xi_{ m ion,obs}$. They find a robust low-mass end to the mass-metallicity relation extending down to $M_\star\sim10^{5.6}-10^{6}\,M_\odot$, with a slope $\gamma=0.35^{+0.02}_{-0.03}$ and evidence for two distinct metal-enrichment pathways (overshoot vs undershoot) in the early universe. A striking case, GLIMPSE-55241, shows dust production and an ionized-gas outflow consistent with supernovae driving the early baryon cycle, suggesting SNe dominate dust production, metal enrichment, and feedback in the earliest galaxies.

Abstract

Ultra-faint galaxies at high-$z$ are fundamental elements of the early galaxy assembly, and spectroscopic characterization of this population is essential to understand the earliest galaxy evolution. Leveraging the ultra-deep JWST/NIRCam and NIRSpec observations of a gravitational lensing field of Abell S1063, taken as part of the GLIMPSE survey, we present spectroscopic properties of 16 galaxies fainter than $M_{\rm UV}=-17$ mag, including the metallicity, dust attenuation, and the ionizing photon production efficiency. The emission lines are generally quite strong, roughly half of which cannot be replicated with standard stellar populations and require an extreme ionizing source. We also identify relatively strong [OIII] emission lines from all sample galaxies, which indicates that the low-mass end of the mass-metallicity relation is extended down to $M_\star\sim10^6\ M_\odot$ at $z\sim6$. The strong [OIII] line detection from the lowest-mass galaxy among the sample ($M_\star\sim10^{5.6}\ M_\odot$) stands in contrast to recent reports of extremely metal-poor galaxy candidates at similar mass and redshift, suggesting that there could be two distinct pathways of the earliest metal enrichment as simulations have predicted. Interestingly, we detect both dust attenuation and galactic outflow in one of the sample galaxies with $M_\star=10^{6.6}\ M_\odot$ at $z=5.5$. All the dust, metal, and outflow contents in this galaxy can be consistently explained by supernovae (SNe), indicative of the key roles of SNe in the earliest galaxy assembly such as dust production, metal enrichment, stellar feedback, and baryon cycle.

Figures (5)

• Figure 1: Rest UV absolute magnitudes vs spectroscopic redshifts of the 16 sample galaxies (red filled circles). Black small circles present the entire galaxies targeted in the GLIMPSE-DDT observations. The selected galaxies are fainter than $M_{\rm UV}=-17$ mag at $z_{\rm spec}>5$.
• Figure 2: The mass-metallicity relation (MZR) at $z\sim6$. Left: the observed MZR in this work (stars) shown with measurements in literature Nakajima2023ApJSCurti2024AAPChemerynska2024ApJTopping2025ApJ, color-coded with the sSFR. Sources lacking either H$\alpha$ or H$\beta$ detection in our sample are shown by dashed stars. The red solid curve is the best fit MZR parameterization (Eqn. \ref{['eqn:MZR']}) at $5<z<8$, while the red shaded area denotes the 16th to 84th percentile range of the posterior. The black curve presents the local MZR, while colored dotted curves show MZR predictions from cosmological simulations (FirstLight, Langan2020MNRAS; Thesan-Zoom, McClymont2025arXiv; FIRE-2, Marszewski2024ApJ; Serra, Pallottini2025AAP). Right: the low-mass end of the MZR. Our observed MZR (stars) are compared with a semi-analytical model including explicit Pop III and Pop II metal enrichment Liu2025arXiv. All star forming galaxies at $z\sim6$ in the simulation are plotted (black dots), and those under the first major burst of star formation are marked with blue circles. The holizontal dashed line corresponds to the maximum metallicity that can be achieved with pure-Pop III enrichments, predicted by theoretical simulations Jaacks2018MNRAS.
• Figure 3: The dust-mass vs stellar-mass plot of dust-obscured galaxies at $z>5$. GLIMPSE-55241 (red star) is compared with other FIR-detected galaxies and JWST/NIRSpec sources observed in other gravitational lensing fields. Predictions by cosmological zoom-in simulations are also plotted.
• Figure 4: Observed (dust-corrected but not $f_{\rm esc}$-corrected) ionizing photon production efficiency $\xi_{\rm ion, obs}$ and the rest UV slope $\beta_{\rm UV}$ of selected galaxies in this work. High-S/N sources in rest UV NIRCam photometry are marked with thick symbols. The photoionization model grids with Cloudy are also shown, varying the stellar age, gas electron density, and escape fraction.
• Figure 5: Outflow galaxy candidate GLIMPSE-55241 among the sample. Top: The full spectrum of GLIMPSE-55241. Multiple emission lines are detected at high S/Ns. Bottom: [O iii]5007 and H$\alpha$ line profiles of GLIMPSE-55241. The blue lines denote the best-fit single Gaussian profiles, and the red dashed lines show the additional broad line component needed to replicate the observed line profile. In the bottom sub-panels, the noise-normalized residuals are shown for narrow-line-only fits and two Gaussian fits. For both [O iii]5007 and H$\alpha$ lines, the two Gaussian models are preferred based on the Bayesian Information Criterion.