Can Lyman-alpha Light Reveal the Secret of Reionization?

Astronomers are trying to understand how the universe 'lit up' after the first stars. A team used data from JWST, the most advanced space telescope, to observe very distant galaxies from a time called the 'epoch of reionization'. This period, shortly after the Big Bang, is when light from the first objects began to break through the dark hydrogen fog that enveloped the universe. Researchers identified Lyman-alpha emission, a glowing radiation emitted by hydrogen, in 14 galaxies so distant that their light had been traveling for over 13 billion years. Eight of these were discovered thanks to the combination of JWST data and Keck spectroscopy. One candidate may be an AGN, a galaxy with a supermassive black hole at its center. Scientists also analyzed the environment of these galaxies, investigating whether they live in denser or less dense regions than average. It appears there is no strong correlation between the visibility of Lyman-alpha light and galaxy density. This suggests other factors, such as star formation rate or the presence of AGNs, may influence the visibility of this emission. The findings help refine models about the origin of reionization and highlight the importance of future observations with broader spectral coverage.

Lyman-alpha (Lyα) light is a hydrogen emission line with a wavelength of 121.6 nm in a vacuum. When an electron in hydrogen transitions from the excited state n=2 to the ground state n=1, it emits a photon with this wavelength. During the epoch of reionization (z ≈ 6–10), the universe was still partially neutral, and Lyα light could be absorbed or scattered by neutral hydrogen, making it difficult to observe. However, in ionized environments or near intense radiation sources (such as massive stars or AGNs), hydrogen is already ionized and Lyα light can propagate freely. The authors used JWST FRESCO data to identify [OIII] emitters at 5007 Å, a reliable tracer of ionized hydrogen. Through Keck/MOSFIRE spectroscopy, they confirmed eight new LAEs at z = 7.0–7.7. Comparing LAE and non-LAE properties reveals minor differences, except in recent star formation rate (SFR10/SFR50 ≈ 1.3) and [OIII] equivalent width (≈1.5 times). This implies that galaxies showing Lyα have a slight preference for environments with more recent and intense star formation. However, the lack of significant differences in UV slope (β), UV magnitude (M_UV), or stellar mass (log M_*) suggests that Lyα visibility is not solely determined by stellar mass or age. The interaction with the AGN candidate may play a key role, as AGNs emit intense ionizing radiation that can create 'channels' through neutral hydrogen, facilitating the escape of Lyα light. Future studies with wide-band spectroscopy and more complete sampling will be essential to precisely map the transition from a neutral to an ionized universe.