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The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST


In the first billion years after the Big Bang, sources of ultraviolet (UV) photons are believed to have ionized intergalactic hydrogen, rendering the Universe transparent to UV radiation. Galaxies brighter than the characteristic luminosity L* (refs. 1,2) do not provide enough ionizing photons to drive this cosmic reionization. Fainter galaxies are thought to dominate the photon budget; however, they are surrounded by neutral gas that prevents the escape of the Lyman-α photons, which has been the dominant way to identify them so far. JD1 was previously identified as a triply-imaged galaxy with a magnification factor of 13 provided by the foreground cluster Abell 2744 (ref. 3), and a photometric redshift of z ≈ 10. Here we report the spectroscopic confirmation of this very low luminosity (≈0.05 L*) galaxy at z = 9.79, observed 480 Myr after the Big Bang, by means of the identification of the Lyman break and redward continuum, as well as multiple 4σ emission lines, with the Near-InfraRed Spectrograph (NIRSpec) and Near-InfraRed Camera (NIRCam) instruments. The combination of the James Webb Space Telescope (JWST) and gravitational lensing shows that this ultra-faint galaxy (MUV = −17.35)—with a luminosity typical of the sources responsible for cosmic reionization—has a compact (≈150 pc) and complex morphology, low stellar mass (107.19M) and subsolar (≈0.6 Z) gas-phase metallicity.

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Fig. 1: A false-colour NIRCam image of the Abell 2744 cluster.
Fig. 2: The SED and NIRSpec spectrum of JD1.
Fig. 3: The morphology of JD1 from JWST NIRCam imaging.
Fig. 4: Spectroscopically confirmed sources and their absolute magnitudes.

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Data availability

All data used in this paper are publicly available through the Mikulski Archive for Space Telescopes (MAST) server with the relevant program IDs (2561 for the NIRCam imaging, 2756 for the NIRSpec spectroscopy). All other data generated throughout the analysis are available from the corresponding author on request or at data are provided with this paper.

Code availability

Our analysis makes use of three primary codes, all of which are publicly available. The photometric redshift analyses were performed with EAzY, the latest version of which (including the templates used here) is available at The data reduction of the NIRCam images were performed with the official STScI JWST pipeline, which can be found here: The NIRSpec data were reduced using the msaexp code, which can be found here: The reduced NIRSpec spectrum was analysed with the gsf code, which is available here: The morphological source-plane reconstruction was done with lenstruction, found here:


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G.R.-B. thanks N. Laporte for valuable conversations regarding the NIRSpec spectrum of the source. This work is based on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme JWST-ERS-1324. We acknowledge financial support from NASA through grant JWST-ERS-1324. A.A., A.M., P.B., C.G., P.R. and E.V. acknowledge financial support through grants PRIN-MIUR 2017WSCC32 and 2020SKSTHZ. A.A. has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 101024195 - ROSEAU. A.Z. and L.J.F. acknowledge support by grant no. 2020750 from the United States–Israel Binational Science Foundation and grant no. 2109066 from the United States National Science Foundation, and by the Ministry of Science & Technology, Israel. C.M. acknowledges support by the VILLUM FONDEN under grant 37459. The Cosmic Dawn Center (DAWN) is funded by the Danish National Research Foundation under grant DNRF140. B.M. acknowledges support from an Australian Government Research Training Programme Scholarship. This research is supported in part by the Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013. J.M.D. acknowledges the support of projects PGC2018-101814-B-100 and MDM-2017-0765. A.V.F. is grateful for support from the Christopher R. Redlich Fund and numerous individual donors. X.W. is supported by the Chinese Academy for Sciences Project for Young Scientists in Basic Research, grant no. YSBR-062. M.B. acknowledges support from the Slovenian Research Agency ARRS through grant N1-0238. R.A.W. acknowledges support from NASA JWST Interdisciplinary Scientist grants NAG5-12460, NNX14AN10G and 80NSSC18K0200 from GSFC. We acknowledge support from the Istituto Nazionale di Astrofisica (INAF) (Italian National Institute for Astrophysics) Large Grant 2022 ‘Extragalactic Surveys with JWST’ (PI Pentericci).

Author information

Authors and Affiliations



G.R.-B. led the NIRSpec data reduction and analysis with input from several co-authors. G.R.-B. and T.T. wrote the paper and developed the main interpretation of the results. A.F., M.C., E.M. and D.P. reduced the NIRCam images and provided the photometry. T.M. performed the spectral fitting and derived the physical parameters. A.A., C.G., P.B. and P.R. developed and updated the reference lens model providing predictions on lensed quantities. A.M. contributed to the construction of the photometric and spectroscopic catalogues for the lens model. A.Z. and L.J.F. constructed the new lens model and wrote the accompanying text. T.N., K.G. and W.C. assisted with NIRSpec data reduction and performed quality checks on the 1D and 2D spectra. E.V., P.R. and L.Y. performed the analysis of the source morphology. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Guido Roberts-Borsani.

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Nature thanks Masami Ouchi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 The P(z) of JD1 from EAzY fits to a variety of photometry.

Each P(z) is constructed through EAzY modelling of three sets of photometric data points. Blue adopts HST photometry only3, orange adopts NIRCam photometry only, and red illustrates results for the combined dataset. The HST photometry provides excellent constraining power blueward of the J-band, but lacks the wavelength coverage in the IR that NIRCam provides to exclude low-z interlopers. As such, even using NIRCam data alone provides far better constraints for z > 9 selections. NB: The photo-z estimated by ref. 3 also incorporates Spitzer/IRAC photometry, not used here.

Source data

Extended Data Table 1 Summary of the NIRCam photometry and the best-fit global properties of JD1

Supplementary information

Source data

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Roberts-Borsani, G., Treu, T., Chen, W. et al. The nature of an ultra-faint galaxy in the cosmic dark ages seen with JWST. Nature 618, 480–483 (2023).

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