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 Fudamoto; Jenny 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

doi:10.1038/s41586-024-07043-6

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

Nature. 2024 Feb;626(8001):975-978. doi: 10.1038/s41586-024-07043-6. Epub 2024 Feb 28.

Authors

Hakim Atek¹, Ivo Labbé², Lukas J Furtak³, Iryna Chemerynska⁴, Seiji Fujimoto⁵, David J Setton⁶, Tim B Miller⁷, Pascal Oesch^{8

9}, Rachel Bezanson⁶, Sedona H Price⁶, Pratika Dayal¹⁰, Adi Zitrin², Vasily Kokorev¹⁰, John R Weaver¹¹, Gabriel Brammer⁹, Pieter van Dokkum¹², Christina C Williams^{13

14}, Sam E Cutler¹¹, Robert Feldmann¹⁵, Yoshinobu Fudamoto^{16

17}, Jenny E Greene¹⁸, Joel Leja^{19

20

21}, Michael V Maseda²², Adam Muzzin²³, Richard Pan²⁴, Casey Papovich^{25

26}, Erica J Nelson²⁷, Themiya Nanayakkara², Daniel P Stark¹⁴, Mauro Stefanon²⁸, Katherine A Suess^{29

30}, Bingjie Wang^{19

20

21}, Katherine E Whitaker^{9

11}

Affiliations

¹ Institut d'Astrophysique de Paris, CNRS, Sorbonne Université, Paris, France. hakim.atek@iap.fr.
² Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne, Victoria, Australia.
³ Physics Department, Ben-Gurion University of the Negev, Be'er Sheva, Israel.
⁴ Institut d'Astrophysique de Paris, CNRS, Sorbonne Université, Paris, France.
⁵ Department of Astronomy, The University of Texas at Austin, Austin, TX, USA.
⁶ Department of Physics and Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA, USA.
⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL, USA.
⁸ Department of Astronomy, University of Geneva, Versoix, Switzerland.
⁹ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Copenhagen, Denmark.
¹⁰ Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands.
¹¹ Department of Astronomy, University of Massachusetts, Amherst, MA, USA.
¹² Department of Astronomy, Yale University, New Haven, CT, USA.
¹³ NSF's National Optical-Infrared Astronomy Research Laboratory, Tucson, AZ, USA.
¹⁴ Steward Observatory, University of Arizona, Tucson, AZ, USA.
¹⁵ Institute for Computational Science, University of Zurich, Zurich, Switzerland.
¹⁶ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, Tokyo, Japan.
¹⁷ National Astronomical Observatory of Japan, Tokyo, Japan.
¹⁸ Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA.
¹⁹ Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA, USA.
²⁰ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA, USA.
²¹ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA, USA.
²² Department of Astronomy, University of Wisconsin, Madison, WI, USA.
²³ Department of Physics and Astronomy, York University, Toronto, Ontario, Canada.
²⁴ Department of Physics and Astronomy, Tufts University, Medford, MA, USA.
²⁵ Department of Physics and Astronomy, Texas A&M University, College Station, TX, USA.
²⁶ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, USA.
²⁷ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO, USA.
²⁸ Departament d'Astronomia i Astrofìsica, Universitat de València, Valencia, Spain.
²⁹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, USA.
³⁰ Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA, USA.

PMID: 38418911
DOI: 10.1038/s41586-024-07043-6

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 Bang^1-3, has been a matter of debate⁴. Some models suggest that high ionizing emissivity and escape fractions (f_esc) from quasars support their role in driving cosmic reionization^5,6. Others propose that the high f_esc 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 f_esc, can effectively dominate cosmic reionization^8,9. 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 M_UV ≈ -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[ξ_ion (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%.