Discovery of the Lyman-α Forest in a Star-Forming Galaxy at z ≈ 7.57: Implications for the Timing and Topology of Cosmic Reionization
Abstract
The escape of ionizing photons from galaxies during the Epoch of Reionization is often inferred indirectly because the intervening neutral hydrogen in the intergalactic medium (IGM) is expected to absorb all flux blueward of Lyman-α at these redshifts. However, in this work, we report the significant detection (≈7σ) of flux blueward of Lyα in a massive (M∗≈109.1 M⊙) star-forming galaxy at z≈7.57, JADES-GS-z7-LAF, using JWST/NIRSpec PRISM spectroscopy. We independently confirm this detection of the Lyα forest (LAF) using JWST/NIRCam F090W imaging (≈10σ). The blueward flux in F090W is spatially coincident with the galaxy and emerges ≈0.4 pkpc from its UV centroid at λrest≈1500 mathringA. We find no evidence for a foreground interloper producing the observed flux, although we cannot formally reject this interpretation with the existing data. The transmitted flux implies an extraordinarily transparent sightline (xHI≪0.1%) through the IGM at z ≈ 5.5−7.5, a sightline that is spatially and kinematically coincident with numerous high-redshift galaxy overdensities, which possibly explains the emergence of this flux. We observe a smooth and gradual turnover around the Lyα break that is either consistent with an extreme damping-wing (NHI≈1023.3 cm−2) or a strong nebular continuum (contributing ≈60−70% of the flux at λrest=1500 mathringA). JADES-GS-z7-LAF is far more luminous (MUV≈−21.5, similar to GN-z11) than the faint sources usually invoked as the drivers of cosmic reionization. If such systems are common in the early Universe, massive galaxies may contribute more to the ionizing photon budget than suggested by other indirect constraints.
Related
- An ultra-faint, chemically primitive galaxy forming in the reionization era
- Resonantly Scattered CIV Emission in Local Star-forming Galaxies: Radiative Transfer Constraints on High-ionization Gas in Reionization Analogs
- Lyman-α forest 1D flux power spectrum constraints on QSO-assisted reionization models
- Forecast for the detectability of patchy hydrogen reionization in WEAVE-QSO measurements of the Lyman-α forest power spectrum at redshift z ≥ 4
- Forecast for the detectability of patchy hydrogen reionization in WEAVE-QSO measurements of the Lyman-α forest power spectrum at redshift z ≥ 4
BibTeX
@misc{helton2026discovery,
title = {{Discovery of the Lyman-$α$ Forest in a Star-Forming Galaxy at $z \approx 7.57$: Implications for the Timing and Topology of Cosmic Reionization}},
author = {Jakob M. Helton and Joris Witstok and Marta Laska and Kevin N. Hainline and Nikko J. Cleri and Francesco D'Eugenio and Ignas Juodžbalis and Nimisha Kumari and Joel Leja and Sandro Tacchella and Rachana Bhatawdekar and Andrew J. Bunker and Daniel J. Eisenstein and Zhiyuan Ji and Benjamin D. Johnson and Pierluigi Rinaldi and Brant E. Robertson and Christopher N. A. Willmer},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2609.38080},
eprint = {2609.38080},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.38080},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Discovery of the Lyman-α Forest in a Star-Forming Galaxy at z ≈ 7.57: Implications for the Timing and Topology of Cosmic Reionization
AU - Jakob M. Helton
AU - Joris Witstok
AU - Marta Laska
AU - Kevin N. Hainline
AU - Nikko J. Cleri
AU - Francesco D'Eugenio
AU - Ignas Juodžbalis
AU - Nimisha Kumari
AU - Joel Leja
AU - Sandro Tacchella
AU - Rachana Bhatawdekar
AU - Andrew J. Bunker
AU - Daniel J. Eisenstein
AU - Zhiyuan Ji
AU - Benjamin D. Johnson
AU - Pierluigi Rinaldi
AU - Brant E. Robertson
AU - Christopher N. A. Willmer
PY - 2026
DA - 2026/10/05
PB - arXiv
DO - 10.48550/arxiv.2609.38080
UR - https://arxiv.org/abs/2609.38080
AB - The escape of ionizing photons from galaxies during the Epoch of Reionization is often inferred indirectly because the intervening neutral hydrogen in the intergalactic medium (IGM) is expected to absorb all flux blueward of Lyman-α at these redshifts. However, in this work, we report the significant detection (≈7σ) of flux blueward of Lyα in a massive (M_∗≈10^9.1 M_⊙) star-forming galaxy at z≈7.57, JADES-GS-z7-LAF, using JWST/NIRSpec PRISM spectroscopy. We independently confirm this detection of the Lyα forest (LAF) using JWST/NIRCam F090W imaging (≈10σ). The blueward flux in F090W is spatially coincident with the galaxy and emerges ≈0.4 pkpc from its UV centroid at λ_rest≈1500 mathringA. We find no evidence for a foreground interloper producing the observed flux, although we cannot formally reject this interpretation with the existing data. The transmitted flux implies an extraordinarily transparent sightline (x_HI≪0.1%) through the IGM at z ≈ 5.5−7.5, a sightline that is spatially and kinematically coincident with numerous high-redshift galaxy overdensities, which possibly explains the emergence of this flux. We observe a smooth and gradual turnover around the Lyα break that is either consistent with an extreme damping-wing (N_HI≈10^23.3 cm^−2) or a strong nebular continuum (contributing ≈60−70% of the flux at λ_rest=1500 mathringA). JADES-GS-z7-LAF is far more luminous (M_UV≈−21.5, similar to GN-z11) than the faint sources usually invoked as the drivers of cosmic reionization. If such systems are common in the early Universe, massive galaxies may contribute more to the ionizing photon budget than suggested by other indirect constraints.
ER -