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

Hakim Atek, Ivo Labbé, Lukas J. Furtak, Iryna Chemerynska, Seiji Fujimoto, David J. Setton, Tim B. Miller, Pascal Oesch, Rachel Bezanson, Sedona H. Price, Pratika Dayal, Adi Zitrin, Vasily Kokorev, John R. Weaver, Gabriel Brammer, Pieter van Dokkum, Christina C. Williams, Sam E. Cutler, Robert Feldmann, Yoshinobu FudamotoJenny E. Greene, Joel Leja, Michael V. Maseda, Adam Muzzin, Richard Pan, Casey Papovich, Erica J. Nelson, Themiya Nanayakkara, Daniel P. Stark, Mauro Stefanon, Katherine A. Suess, Bingjie Wang, Katherine E. Whitaker

Research output: Contribution to journalArticlepeer-review

7 Scopus citations


The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800 Myr after the Big Bang1–3, 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%.

Original languageEnglish (US)
Pages (from-to)975-978
Number of pages4
Issue number8001
StatePublished - Feb 29 2024

All Science Journal Classification (ASJC) codes

  • General


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