An international team of researchers analyzing data from NASA’s James Webb Space Telescope has identified a galaxy that emitted Lyman-α radiation when the Universe was just 330 million years old. This detection, which marks the earliest known sign of hydrogen reionization, has significant implications for understanding how the first stellar populations and possible black holes emerged in the early Universe.
The galaxy, designated JADES-GS-z13-1-LA, was observed using Webb’s Near Infrared Spectrograph (NIRSpec) as part of the JWST Advanced Deep Extragalactic Survey (JADES). Researchers observed a distinct emission line corresponding to Lyman-α radiation at a redshift of z = 13.05 ± 0.01, representing a time when the intergalactic medium was largely composed of neutral hydrogen gas. This signal challenges expectations, as ultraviolet radiation from that period should have been heavily absorbed by surrounding neutral gas unless the galaxy had already carved out a large ionized region around itself.
The Lyman-α emission was observed alongside an unusually blue ultraviolet continuum, indicating the presence of extremely hot and young stars or, alternatively, an active galactic nucleus. The data suggest that this object either hosted a population of unusually massive stars with surface temperatures exceeding 100,000 K, or it was powered by material accreting onto a compact, highly energetic source such as a black hole.
Deep imaging and spectroscopy were obtained over nearly 19 hours of exposure. The line detection, appearing at a wavelength of 1.7084 μm in the observed frame, stood out with a signal-to-noise ratio of 6.4. The measured rest-frame equivalent width for Lyman-α exceeded 40 Å, a strength not typically seen at such high redshifts. This suggests that the galaxy was highly efficient at producing and leaking ionizing photons, which helped create a surrounding bubble of ionized hydrogen, allowing the radiation to escape.
Detailed modeling of the spectrum points to two main interpretations. The first posits that the emission originates from very massive, low-metallicity stars, potentially as much as 300 times the mass of the Sun, which would account for the intense output of ionizing radiation. The second possibility is the presence of a growing supermassive black hole generating radiation via accretion. The spatial compactness of the observed emission, with a half-light radius smaller than 35 parsecs, aligns with expectations for an active nucleus or a dense stellar cluster.
Statistical models further suggest that JADES-GS-z13-1-LA is embedded in a localized ionized region, at least 200,000 parsecs across. This distance would have allowed the Lyman-α photons to redshift sufficiently to avoid being absorbed by the surrounding neutral hydrogen. Calculations estimate the intrinsic luminosity of the Lyman-α emission at approximately 3 × 10⁴³ erg/s. Without a surrounding ionized bubble, the necessary intrinsic brightness would have been three times higher, a scenario that is less favored due to the lack of associated emission detected in mid-infrared bands.
The photometric data also support the finding. The galaxy’s ultraviolet slope was measured to be steeper than −2.7, consistent with a very blue and relatively dust-free source. This color profile supports the presence of massive stars with high temperatures or a non-thermal continuum from a black hole-driven source. Either scenario leads to a high production rate of hydrogen-ionizing photons.
Researchers used several spectral models, including pure power-law continua and nebular emission scenarios, but found that the data were best fit by a steep power-law slope, suggesting a minimal contribution from nebular free-bound and free-free continuum emission. Additional analysis ruled out simpler cooling radiation from cloud collapse as a dominant mechanism for the Lyman-α signature.
While the possible contribution from Population III stars was considered, the total mass of the stellar population and the absence of strong helium II emission lines made it unlikely that the object is composed purely of these metal-free stars. Population III stars are expected to be hotter and more massive than later stellar generations, and they emit a large fraction of their luminosity in Lyman-α radiation. However, the galaxy’s properties don’t align neatly with expectations for such a pristine population.
Another hypothesis involves a damped Lyman-α absorber (DLA) within the galaxy’s own interstellar medium. The spectrum shows a smooth ultraviolet turnover, possibly caused by a dense, neutral hydrogen column near the galaxy that absorbs shorter-wavelength light. Under this model, the geometry must allow Lyman-α photons to escape, perhaps through an ionization cone or a patchy, inhomogeneous gas distribution, while the continuum source remains obscured.
If the emission originates from a black hole accretion disk, its properties align with known behaviors of active galactic nuclei. Such disks can produce steep ultraviolet slopes and large escape fractions for ionizing radiation. In this scenario, the observed broad Lyman-α line may be explained by energetic outflows or emission from a broad line region. Constraints on accompanying emission lines such as He II remain inconclusive, but they do not rule out the possibility of a compact nucleus.
The galaxy appears spatially unresolved in the available imaging data, indicating an extremely compact source. Observations using the Near Infrared Camera (NIRCam) place an upper limit of 35 parsecs on its half-light radius. This is smaller than typical galaxies observed at similar redshifts, suggesting either a dense stellar cluster or a compact active nucleus.
Lyman-α radiation is a key marker for reionization. During the early period of the Universe, neutral hydrogen absorbed most ultraviolet radiation, making direct detection of early galaxies difficult. As stars and galaxies formed, they began ionizing their surroundings. By detecting this line from a source so early in cosmic history, researchers have gained a clearer view of when and how this process began.
While this observation does not alone resolve questions about the reionization timeline, it contributes important data. Until now, most models placed the bulk of hydrogen reionization between 400 million and one billion years after the Big Bang. This detection suggests that the process may have begun earlier and in more localized patches, driven by individual galaxies or small groups of them.
It also raises the question of how galaxies at such an early time could have assembled enough mass and sustained high rates of ionizing radiation. Either their stars formed rapidly and were unusually massive, or black holes began growing and contributing to their host galaxy’s radiation budget far earlier than previously demonstrated.
The presence of such an early ionized bubble is notable given the surrounding medium’s likely neutral state. For Lyman-α photons to escape absorption, they must redshift far enough from their emission wavelength before encountering neutral hydrogen. This requires either a long distance traveled within an ionized region or a substantial velocity offset. The emission profile of JADES-GS-z13-1-LA suggests both conditions may have been met.
The researchers accounted for this in their models, calculating that only a fraction of the emitted Lyman-α photons would survive their journey through space. Their estimates placed the local escape fraction near unity, with the effective production efficiency of ionizing photons, ξion, reaching values that challenge standard stellar population models. Some hypotheses allow for such efficiencies by invoking non-standard initial mass functions skewed toward more massive stars.
The scenario becomes more complex when accounting for the Lyman continuum escape fraction, the portion of high-energy photons that avoid absorption within the galaxy. A near-unity escape fraction is rare but consistent with the observed steep ultraviolet slope and absence of strong nebular features. The models also suggest that without continuous ionizing output, the ionized region would recombine in less than a few million years.
While more such detections will be needed to build a statistically meaningful picture, this discovery shows that isolated galaxies at very high redshift can play a role in early reionization. It also emphasizes the importance of JWST’s spectroscopic capabilities. Earlier observations using the Hubble Space Telescope lacked the sensitivity and wavelength coverage needed to detect faint Lyman-α lines from such early times.
The effort required to identify and confirm JADES-GS-z13-1-LA’s properties was significant. Researchers invested over 18 hours of JWST time to obtain the required data. The detection appeared consistently across two independent observations, reinforcing the reliability of the finding. However, its spectral resolution was limited by the PRISM mode’s settings, which favored sensitivity over detail.
Upcoming JWST programs may attempt follow-up observations using higher-resolution settings or focus on similar targets to determine whether this galaxy is typical or an outlier. Either outcome would refine current theories of early star and black hole formation.
This discovery marks a critical step in uncovering the physical conditions and formation mechanisms of the Universe’s first luminous structures. As JWST continues to probe deeper into cosmic time, researchers expect to encounter more sources like JADES-GS-z13-1-LA, further clarifying the sequence of events that lit up the Universe after the Big Bang.
Source:
Witstok, J. et al. (2025). Witnessing the onset of reionization through Lyman-α emission at redshift 13. Nature, 639, 870–872. https://doi.org/10.1038/s41586-025-08779-5
