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

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 ≲ z ≲ 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 MUV ≲ −10 galaxies in the overdensity, provided they have log ₁₀ ξ ᵢₒₙ fesc / \ [erg⁻¹Hz] 25 (e.g. log ₁₀ξ ᵢₒₙ / \ [erg⁻¹Hz]=25.4 and fesc = 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.