Probing patchy reionisation with JWST: IGM opacity constraints from the Lyman-$α$ forest of galaxies in legacy extragalactic fields

Romain A. Meyer, Guido Roberts-Borsani, Pascal Oesch, Richard S. Ellis

First listed 2025-04-03 | Last updated 2025-08-28

Abstract

We present the first characterization of the Gunn-Peterson trough in high-redshift galaxies using public JWST NIRSpec spectroscopy. This enables us to derive the first galaxy-based IGM opacity measurements at the end of reionisation. Using galaxy spectra has several advantages over quasar spectra: it enables measurements of the IGM opacity in any extragalactic field over a continuous redshift range $4\lesssim z\lesssim 7$, as well as measurements of the intrinsic Lyman-$β$ opacity. Our novel constraints are in good agreement with state-of-the-art ground-based quasar Lyman-$α$ forest observations, and will become competitive as the number of JWST $z>5$ galaxy spectra rapidly increases. We also provide the first constraints on the uncontaminated Lyman-$β$ opacity at $5<z<6$. Finally, we demonstrate the power of JWST to connect the ionisation state of the IGM to the sources of reionisation in a single extragalactic field. We show that a previously reported galaxy overdensity and an excess of Lyman-$α$ emitters detected with JWST in GOODS-South at $z=5.8-5.9$ coincides with an anomalously low IGM opacity to Lyman-$α$ at this redshift. The local photo-ionisation rate excess can be fully accounted for by the cumulative ionising output of $M_{\rm{UV}}\lesssim -10$ galaxies in the overdensity, provided they have $\log_{10}\langle ξ_{\rm{ion}} f_{\rm{esc}} / \ [\rm{erg}^{-1}\rm{Hz}]\rangle \simeq 25$ (e.g. $\log_{10}ξ_{\rm{ion}} / \ [\rm{erg}^{-1}\rm{Hz}]=25.4$ and $f_{\rm{esc}}=40\%$). Overall, this breakthrough offers a new way to connect the galaxy large-scale structure to the state of the IGM, potentially enabling us to precisely identify the sources of reionisation.

Short digest

Using public JWST/NIRSpec PRISM spectra of z≈4–7 galaxies, the authors detect the Gunn–Peterson trough in stacked galaxy spectra and derive the first galaxy-based IGM Lyα opacity measurements, plus intrinsic Lyβ opacity at 5<z<6. The galaxy-based opacities agree with state-of-the-art quasar Lyman-α forest constraints and will sharpen as JWST samples grow. In GOODS-South, a z=5.8–5.9 overdensity of galaxies and Lyα emitters coincides with an anomalously low Lyα IGM opacity, directly linking local large-scale structure to an over-ionised IGM patch. The required photo-ionisation rate can be produced by galaxies down to M_UV≲−10 if log10⟨ξion fesc⟩≈25 (e.g., log10 ξion=25.4 with fesc=0.4).

Key figures to inspect

• Figure 1: Inspect the stacked, normalised spectra to see the onset of the Gunn–Peterson trough blueward of Lyα and any damping-wing signature; check the systemic Lyα/Lyβ markers (from rest-optical lines) and the stack errors to gauge the robustness of the trough detection across redshift bins.
• Figure 2: Compare the galaxy-derived Lyα transmission/opacity versus redshift to the colored quasar measurements; note the continuous 4≲z≲7 coverage and how single-field (gray) points scatter around the stacked (black) values, highlighting emerging field-to-field variance.
• Figure 3: Use the cumulative distribution of τeff(Lyα) to place each field (GOODS-S, GOODS-N, EGS, Abell 2744) within the Bosman+2022 distributions; GOODS-S should sit at unusually low τeff, quantifying the over-ionised patch relative to typical sightlines.
• Figure 4: Top panel shows observed τeff in Lyβ including foreground Lyα contamination—verify how GOODS-S dominates the stack; bottom panel applies the foreground-galaxy correction to recover intrinsic Lyβ opacity and compare it to Lyα-based expectations and IGM parameter bands.