Characterize the intrinsic Lyman-α spectral profile at high redshift (z ≥ 7)

Characterize the intrinsic Lyman-α spectral profile emerging from galaxies at redshift z ≥ 7, explicitly accounting for the fact that asymmetric intergalactic medium transmission on the blue side of the line prevents direct reconstruction from observed spectra.

Background

In interpreting the detected Lyman-α emission from JADES-GS-z13-1-LA at z ≈ 13, the authors emphasize that the intergalactic medium (IGM) at z > 7 strongly and asymmetrically attenuates the blue side of Lyman-α. This prevents straightforward inference of the intrinsic line profile from the observed spectrum.

Although their modelling and the R1000 non-detections suggest the presence of a broad, redshifted component to enable IGM transmission, the exact emergent Lyman-α profile remains unconstrained by current data. Establishing this intrinsic profile is essential for robustly inferring systemic redshifts, outflow kinematics, ionised bubble properties, and the production and escape of ionising photons at Cosmic Dawn.

References

While the emergent Ly spectral profile is fundamentally unknown at z ≳ 7 due to the asymmetric IGM transmission on the blue side, some clues are given by the non-detection of the line in the R1000 spectra.

— Witnessing the onset of reionisation via Lyman-$α$ emission at redshift 13  (2408.16608 - Witstok et al., 2024) in Methods, Spectral modelling (paragraph beginning “Radiative transfer calculations predict a large variety of Ly spectral profiles…”)

We note that the existing data make it hard to determine whether these are smooth and continuous transmission windows at $z \approx 5.5-7.5$ or a jagged forest of highly transmissive windows surrounded by complete absorption troughs, which motivates deeper and higher-resolution follow-up spectroscopy of JADES-GS-z7-LAF and its surrounding large-scale environment with JWST/NIRSpec.

— 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  (2609.38080 - Helton et al., 29 Sep 2026) in Section 3.9, “Spatial-Spectral Correlation for Transmission”

We test for this using the following methodology and determine that the likelihood of the observed flux coming from a foreground interloper is negligible, although we cannot formally reject this interpretation with the existing data.

— 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  (2609.38080 - Helton et al., 29 Sep 2026) in Section 3.5, “Testing for Foreground Interlopers”