Most of the photons that reionized the Universe came from dwarf galaxies.
basic_science · Level V
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- Record sourced from PubMed, PMID 38418911.
- Also identified by DOI 10.1038/s41586-024-07043-6.
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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 Bang<sup>1-3</sup>, has been a matter of debate<sup>4</sup>. Some models suggest that high ionizing emissivity and escape fractions (f<sub>esc</sub>) from quasars support their role in driving cosmic reionization<sup>5,6</sup>. Others propose that the high f<sub>esc</sub> values from bright galaxies generate sufficient ionizing radiation to drive this process<sup>7</sup>. 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<sub>esc</sub>, can effectively dominate cosmic reionization<sup>8,9</sup>. 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<sub>UV</sub> ≈ -17 mag and -15 mag (down to 0.005L<sup>⋆</sup> (refs. <sup>10,11</sup>)). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξ<sub>ion</sub> (Hz erg<sup>-1</sup>)] = 25.80 ± 0.14, a factor of 4 higher than commonly assumed values<sup>12</sup>. 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%.