A new analysis of JWST spectra from more than 1,400 galaxies indicates that cosmic reionization — the last major phase transition in the history of the universe — may have been driven by a surprisingly small subset of galaxies rather than the population as a whole. The work, led by Emma Giovinazzo (University of Geneva), was posted to arXiv on July 24, 2026.
The reionization budget
Roughly 600 million to a billion years after the Big Bang, ultraviolet radiation from early stars and galaxies ionized the neutral hydrogen that filled the young universe, converting it to the ionized state we observe today. The radiation responsible is dominated by Lyman continuum (LyC) photons, with wavelengths below 912 Å. Whether these photons actually reach the intergalactic medium depends on two quantities: how efficiently galaxies produce them, and what fraction manages to escape rather than being absorbed internally by gas and dust.
The central question Giovinazzo’s team set out to answer was which kind of galaxy does the work — a few galaxies that leak ionizing photons very efficiently, or many galaxies that each leak only a little.
Driven by the few
The team compiled JWST/NIRSpec spectra for 1,428 galaxies spanning redshift 5 to 10, and for each one estimated both the ionizing-photon production rate and the fraction escaping into the intergalactic medium. The galaxies split cleanly into two groups: about 20% are «strong leakers,» with escape fractions above 10%, while the rest leak weakly. Despite being the minority, the strong leakers produce roughly 87% of the ionizing photons that reach the intergalactic medium — enough, on their own, to account for nearly all of reionization.
The relative contributors also shift with cosmic time. At lower redshift (z = 5–6), bright galaxies dominate the ionizing output; at higher redshift (z > 6), faint galaxies contribute roughly on par with brighter ones. Combining these results, the team estimates reionization was essentially complete by z ≈ 5.8, consistent with independent constraints from the cosmic microwave background and quasar absorption studies.
The caveats
The authors are explicit about the limits of the result. Their precise numbers depend on modeling assumptions and on a single survey covering a small patch of sky, and the extrapolation to the faintest galaxies remains unverified — which they flag as their biggest caveat. Resolving these uncertainties, they note, will require many more extremely deep JWST fields, or observations with the upcoming Extremely Large Telescope. The practical takeaway is pointed: if a minority of strong leakers drove reionization, then identifying those specific galaxies — not simply counting the population — is what will pin down how the universe was reionized.
© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Emma Giovinazzo et al., Reionization driven by the few: the ionizing budget of galaxies at z=5-10 from JWST/NIRSpec, arXiv:2607.22834 (2026). DOI: 10.48550/arXiv.2607.22834
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, analyzing 1,428 galaxies at redshift 5 to 10, suggests cosmic reionization was driven by a minority of "strong leaker" galaxies. Although only about 20% of galaxies let more than 10% of their ionizing radiation escape, these produced roughly 87% of the Lyman continuum photons reaching the intergalactic medium — enough to drive nearly all reionization, complete by z ≈ 5.8.){kind=link}