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Most of the photons that reionized the Universe came from dwarf galaxies
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Journal Article
Most of the photons that reionized the Universe came from dwarf galaxies
2024
Overview
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 debate
4
. 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 process
7
. 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 0.005
L
⋆
(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 values
12
. 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%.
An analysis of eight ultra-faint galaxies during the epoch of reionization with absolute magnitudes between −17 mag and −15 mag shows that most of the photons that reionized the Universe come from dwarf galaxies.
