JWST study suggests a handful of 'leaky' galaxies reionized the early universe

A study analyzing data from the James Webb Space Telescope (JWST) suggests that a small number of "leaky" galaxies were primarily responsible for cosmic reionization. The research, led by Emma Giovinazzo from the University of Geneva, used JWST/NIRSpec spectra from 1,428 galaxies observed at redshifts 5–10, corresponding to roughly 500 million to 1 billion years after the Big Bang.

The team determined that 20% of these galaxies are strong leakers, emitting Lyman-continuum (LyC) photons with wavelengths under 912 Å with escape fractions above 10%. Despite their minority status, these strong leakers account for 87% of the ionizing photons reaching the intergalactic medium, effectively driving the reionization process.

The study also found that galaxies' contribution to reionization evolves over cosmic time—bright galaxies dominated the process at lower redshifts (z=5–6), while fainter galaxies played a more significant role at higher redshifts (z>6). Based on their analysis, the researchers estimate that reionization was completed by around z ~ 5.8.

However, the precision of these findings depends on modeling choices and the limited sky coverage of the survey, with the team highlighting the need for further deep field observations with JWST or the upcoming Extremely Large Telescope (ELT) to resolve these uncertainties.