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Most of the photons that reionized the Universe came from dwarf galaxies

Nature volume 626, pages 975–978 (2024)Cite this article

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Abstract

The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800 Myr after the Big Bang1,2,3, has been a matter of debate4. Some models suggest that high ionizing emissivity and escape fractions (fesc) from quasars support their role in driving cosmic reionization5,6. Others propose that the high fesc values from bright galaxies generate sufficient ionizing radiation to drive this process7. Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and fesc, can effectively dominate cosmic reionization8,9. However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between MUV ≈ −17 mag and −15 mag (down to 0.005L⋆ (refs. 10,11)). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξion (Hz erg−1)] = 25.80 ± 0.14, a factor of 4 higher than commonly assumed values12. If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.

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Fig. 1: Layout of the ultra-faint galaxies identified in A2744 cluster field.
Fig. 2: Spectroscopic observations of the faintest galaxies during the epoch of reionization.
Fig. 3: The ionizing photon production efficiency of faint galaxies during the epoch of reionization.
Fig. 4: The total ionizing emissivity of galaxies at z ~ 7.

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Data availability

The NIRCam and HST imaging data are available on the UNCOVER webpage at GitHub (https://jwst-uncover.github.io/). The NIRSpec spectroscopic data are publicly available through the Mikulski Archive for Space Telescopes (MAST; https://archive.stsci.edu/), under programme ID 2561. The UNCOVER lensing products are available at GitHub (https://jwst-uncover.github.io/DR1.html#LensingMaps).

Code availability

The following codes were used in this study: Astropy100,101, Bagpipes58,59, BEAGLE69, EAzY49, Matplotlib102, msaexp v.0.6.10103, NumPy104, NUTS97,105, PyMultinest67,68, pysersic95, SciPy106 and GrizLi (https://github.com/gbrammer/grizli).

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Acknowledgements

H.A. and I.C. acknowledge support from CNES, focused on the JWST mission and the Programme National Cosmology and Galaxies (PNCG) of CNRS/INSU with INP and IN2P3, co-funded by CEA and CNES. H.A. thanks the Cosmic Dawn Center (DAWN) for their support. DAWN is funded by the Danish National Research Foundation (grant no. 140). I.L. acknowledges support from the Australian Research Council through Future Fellowship FT220100798. P.D. acknowledges support from the NWO (grant no. 016.VIDI.189.162) (ODIN) and from the CO-FUND Rosalind Franklin programme of the European Commission and the University of Groningen. A.Z. acknowledges support from the US–Israel Binational Science Foundation (BSF) (grant no. 2020750), the US National Science Foundation (NSF) (grant no. 2109066) and the Ministry of Science and Technology, Israel. The work of C.C.W. is supported by NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the NSF.

Author information

Authors and Affiliations

  1. Institut d’Astrophysique de Paris, CNRS, Sorbonne Université, Paris, France

    Hakim Atek & Iryna Chemerynska

  2. Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne, Victoria, Australia

    Ivo Labbé, Adi Zitrin & Themiya Nanayakkara

  3. Physics Department, Ben-Gurion University of the Negev, Be’er Sheva, Israel

    Lukas J. Furtak

  4. Department of Astronomy, The University of Texas at Austin, Austin, TX, USA

    Seiji Fujimoto

  5. Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA, USA

    David J. Setton, Rachel Bezanson & Sedona H. Price

  6. Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA

    Tim B. Miller

  7. Department of Astronomy, University of Geneva, Versoix, Switzerland

    Pascal Oesch

  8. Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark

    Pascal Oesch, Gabriel Brammer & Katherine E. Whitaker

  9. Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands

    Pratika Dayal & Vasily Kokorev

  10. Department of Astronomy, University of Massachusetts, Amherst, MA, USA

    John R. Weaver, Sam E. Cutler & Katherine E. Whitaker

  11. Department of Astronomy, Yale University, New Haven, CT, USA

    Pieter van Dokkum

  12. NSF’s National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA

    Christina C. Williams

  13. Steward Observatory, University of Arizona, Tucson, AZ, USA

    Christina C. Williams & Daniel P. Stark

  14. Institute for Computational Science, University of Zurich, Zurich, Switzerland

    Robert Feldmann

  15. Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, Tokyo, Japan

    Yoshinobu Fudamoto

  16. National Astronomical Observatory of Japan, Tokyo, Japan

    Yoshinobu Fudamoto

  17. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA

    Jenny E. Greene

  18. Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, USA

    Joel Leja & Bingjie Wang

  19. Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA, USA

    Joel Leja & Bingjie Wang

  20. Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, USA

    Joel Leja & Bingjie Wang

  21. Department of Astronomy, University of Wisconsin, Madison, WI, USA

    Michael V. Maseda

  22. Department of Physics and Astronomy, York University, Toronto, Ontario, Canada

    Adam Muzzin

  23. Department of Physics and Astronomy, Tufts University, Medford, MA, USA

    Richard Pan

  24. Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA

    Casey Papovich

  25. George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA

    Casey Papovich

  26. Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO, USA

    Erica J. Nelson

  27. Departament d’Astronomia i Astrofìsica, Universitat de València, Valencia, Spain

    Mauro Stefanon

  28. Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA

    Katherine A. Suess

  29. Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA, USA

    Katherine A. Suess

Authors
  1. Hakim Atek
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  2. Ivo Labbé
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  3. Lukas J. Furtak
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  4. Iryna Chemerynska
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  5. Seiji Fujimoto
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  6. David J. Setton
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  7. Tim B. Miller
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  8. Pascal Oesch
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  9. Rachel Bezanson
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  10. Sedona H. Price
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  11. Pratika Dayal
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  12. Adi Zitrin
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  13. Vasily Kokorev
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  14. John R. Weaver
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  15. Gabriel Brammer
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  16. Pieter van Dokkum
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  17. Christina C. Williams
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  18. Sam E. Cutler
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  19. Robert Feldmann
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  20. Yoshinobu Fudamoto
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  21. Jenny E. Greene
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  22. Joel Leja
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  23. Michael V. Maseda
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  24. Adam Muzzin
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  25. Richard Pan
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  26. Casey Papovich
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  27. Erica J. Nelson
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  28. Themiya Nanayakkara
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  29. Daniel P. Stark
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  30. Mauro Stefanon
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  31. Katherine A. Suess
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  32. Bingjie Wang
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  33. Katherine E. Whitaker
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Contributions

H.A. led the analysis and writing of the paper. L.J.F. and A.Z. constructed the lens model and extracted lensing-related quantities. S.F. produced the figures. I.L. and R.B. are the principal investigators of the UNCOVER programme. R.B. and I.L. designed the observations and reduced the spectra. J.R.W. and B.W. produced the catalogues used for target selection. P.D. provided simulations to interpret the observational results obtained. V.K. produced line measurements. I.C. estimated survey volumes. D.J.S. ran an SED fitting analysis. T.B.M. measured the galaxy sizes. All authors contributed to the paper and aided the analysis and interpretation.

Corresponding author

Correspondence to Hakim Atek.

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The authors declare no competing interests.

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Nature thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 UNCOVER JWST data for galaxy 16155 at zspec = 6.88.

The top panels show image cutouts in seven different filters at increasing wavelength including ancillary HST/ACS data in F814W, and UNCOVER JWST imaging in F115W, F150W, F200W, F277W, F356W, and F444W bands (left to right). The central panel shows the UNCOVER NIRSpec data, with the 2D spectrum on top of the 1D optimally extracted spectrum (black with gray 1-σ uncertainty ranges). The red lines show the best-fit msaexp template spectrum. The observed-frame wavelengths of key emission lines are indicated as vertical dashed lines. The bottom panels show a zoomed in version of three different parts of the spectrum around the Lyα break (left), around the [Oiii]+Hβ emission lines (middle) and the Hα line (right).

Extended Data Fig. 2 Stellar population simultaneous fitting to the NIRSpec spectra and NIRCam photometry.

Panel a: Two representative sources (IDs 18924 and 16155) are shown. The best-fit Bagpipes model (red curve) is plotted over the observed NIRSpec spectrum (black curve), together with the error spectrum (gray curve). The NIRCam photometric measurements are represented with black points with their associated 1-sigma uncertainties. Panel b: Posterior distribution function for the main physical properties of ID 16155. When relevant, the parameters are corrected for magnification. Panel c: Same as panel b, for source ID 18924.

Extended Data Fig. 3 Spectroscopic constraints on the UV luminosity function.

The UV luminosity function as determined from our spectroscopic sample is represented by orange points. Also shown, the photometric determination from the HFF data24, together with the best-fit Schechter function (blue curve) and a modified Schechter with a potential turnover (teal curve). The shaded region of each curve represent the 1 − σ uncertainties.

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Atek, H., Labbé, I., Furtak, L.J. et al. Most of the photons that reionized the Universe came from dwarf galaxies. Nature 626, 975–978 (2024). https://doi.org/10.1038/s41586-024-07043-6

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