A UV-Luminous Galaxy at z=11 with Surprisingly Weak Star Formation Activity

Abstract

One of the major discoveries by the James Webb Space Telescope (JWST) is the identification of a large population of luminous galaxies at $z>10$, challenging theoretical models for early galaxy formation. The unexpectedly high number density of these systems has triggered intense debate about potential differences in the physical properties of galaxies at such extreme redshifts and those at lower redshift. However, progress has been limited by the lack of rest-frame optical diagnostics, which are critical for constraining the key properties. Here we present deep JWST/MIRI observations of a UV-luminous galaxy at $z=11.04$, CEERS2-588, only 400 Myr after the Big Bang. CEERS2-588 is detected in the MIRI F560W and F770W bands, while deep MIRI/MRS spectroscopy yields no detection of H$α$ or [OIII]$\lambda5007$ line, revealing a prominent Balmer break detected for the first time at $z>10$. Spectral energy distribution (SED) fitting indicates an extended star formation history possibly reaching $z>15$, followed by rapid quenching within the recent $\sim10$ Myr, in stark contrast to other $z>10$ galaxies. The MIRI detections also significantly improve our stellar mass estimate to $\mathrm{log}(M_*/M_\odot)=9.1^{+0.1}_{-0.1}$, making CEERS2-588 the most massive galaxy securely confirmed at $z>10$. Remarkably, the inferred gas-phase metallicity is near solar, exceeding predictions from current theoretical models. These results suggest that efficient starbursts play a key role in producing the abundant luminous galaxy population in the early universe.

Figures (7)

• Figure 1: JWST MIRI observations of CEERS2-588. Top panels show 6$^{\prime\prime}$$\times$6$^{\prime\prime}$ cutout images obtained with JWST NIRCam and MIRI, with $\sim6\sigma$ detections in the MIRI F560W and F770W bands. The bottom panels present the extracted one-dimensional MIRI/MRS spectra (black), with $1\sigma$ uncertainties indicated in grey, around the expected wavelengths of the [O iii]$\lambda5007$ and H$\alpha$ lines. Inset panels show the MIRI/MRS maps at these wavelengths, integrated over $\pm100$ km s$^{-1}$. No significant [O iii]$\lambda5007$ or H$\alpha$ emission is detected in the spectra, indicating that the MIRI F560W and F770W detections are dominated by rest-frame optical stellar continuum emission.
• Figure 1: JWST NIRCam, MIRI, and NIRSpec datasets of CEERS2-588. Top panels show 4$^{\prime\prime}$$\times$4$^{\prime\prime}$ cutout images obtained with JWST NIRCam and MIRI. Middle panels present the NIRSpec two-dimensional spectrum and the MIRI/MRS maps at the expected wavelengths of [O iii]$\lambda5007$ and H$\alpha$. Bottom panels show the extracted one-dimensional NIRSpec and MIRI/MRS spectra (black), with the corresponding $1\sigma$ uncertainties (grey). The NIRSpec spectrum clearly reveals the [O ii]$\lambda3727$ emission line, indicating the spectroscopic redshift of $z_\mathrm{spec}=11.04$.
• Figure 2: Results of SED fitting for CEERS2-588. Left panel shows the SED. Red circles indicate the observed photometric measurements, while the grey line and filled circles show the smoothed NIRSpec spectrum and its binned constraints. Both the NIRCam and MIRI photometry and NIRSpec spectroscopy indicates the existence of a prominent Balmer break. The blue line and open circles represent the best-fitting model. Middle panel compares the observed and model-predicted emission-line fluxes for H$\alpha$, [O iii]$\lambda5007$, and [O ii]$\lambda3727$, demonstrating good agreement between the model and the data. Right panel presents the inferred star formation history. CEERS2-588 undergoes rapid quenching within the recent $\sim10$ Myr, consistent with star formation rates inferred from H$\alpha$ (grey solid line) and UV (grey dashed line) emission, which probe characteristic timescales of $\sim5$ Myr and $\sim100$ Myr, respectively.
• Figure 2: Constraints on the gas-phase metallicity of CEERS2-588. Each panel shows a strong-line ratio used as a metallicity diagnostic as a function of oxygen abundance: $\mathrm{R2}=$[O ii]$\lambda3727$/H$\beta$ (left), $\mathrm{O32}=$[O iii]$\lambda5007$/[O ii]$\lambda3727$ (middle), and $\mathrm{R23}=($[O iii]$\lambda4959,5007+$[O ii]$\lambda3727)$/H$\beta$ (right). Red lines and shaded regions indicate the constraints on the line ratios of CEERS2-588 with $1\sigma$ uncertainties. Grey solid curves show empirical relations for massive galaxies 2017MNRAS.465.1384C2020MNRAS.491..944C, while grey dashed curves correspond to relations for young galaxies at $z \sim 0$2022ApJS..262....3N. Grey squares represent stacked spectra of $z \sim 0$ SDSS galaxies 2017MNRAS.465.1384C. The combined strong-line constraints indicate a gas-phase metallicity of $12+\log(\mathrm{O/H}) \simeq 8.6$ for CEERS2-588, significantly higher than those measured for other galaxies at $z>10$ (blue squares) 2025arXiv251219695H2024ApJ...975..245C2025AA...695A.250A2024ApJ...973...81H.
• Figure 3: Basic properties of CEERS2-588 in comparison with other galaxies and theoretical models. Left panel shows stellar mass as a function of redshift. CEERS2-588 (red diamond) is the most massive galaxy known at $z>10$ among spectroscopically confirmed galaxies (blue squares) 2025arXiv250511263N2025NatAs...9..729H2024Natur.633..318C2025Natur.639..897W2023NatAs...7..622C2023ApJ...957L..34W2025arXiv250722888W2025arXiv251103035C2023Natur.622..707A2025ApJ...988L..10K2025arXiv251202997C2024ApJ...973....8H2025NatAs...9..155Z. Black curves indicate the stellar mass expected for the most massive dark matter halo within the CEERS survey volume at each redshift ($\mathrm{log}(M_\mathrm{h}/M_\odot)=10.8$ at $z=11$) for different assumed integrated star formation efficiencies, while the grey shaded region marks values forbidden by the $\Lambda$CDM cosmology. These curves are shown to illustrate the relative extremeness of CEERS2-588 within the CEERS volume, while the comparison galaxies are not restricted to the CEERS field. Right panel shows the mass-metallicity relation. CEERS2-588 is the most metal-rich galaxy identified at $z>10$, with a metallicity $\sim5-10$ times higher than that of other galaxies with metallicity measurements at similar redshifts 2025arXiv251219695H2024ApJ...975..245C2025AA...695A.250A2024ApJ...973...81H2025NatAs...9..155Z. Its metallicity also exceeds most theoretical predictions at $z \sim 11$ (Methods), indicating rapid metal enrichment in the early Universe.