Probing patchy reionisation with JWST: IGM opacity constraints from the Lyman-$α$ forest of galaxies in legacy extragalactic fields
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2025
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| author | Meyer, Romain A. Roberts-Borsani, Guido Oesch, Pascal Ellis, Richard S. |
| author_facet | Meyer, Romain A. Roberts-Borsani, Guido Oesch, Pascal Ellis, Richard S. |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2504_02683 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Probing patchy reionisation with JWST: IGM opacity constraints from the Lyman-$α$ forest of galaxies in legacy extragalactic fields Meyer, Romain A. Roberts-Borsani, Guido Oesch, Pascal Ellis, Richard S. Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics 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. |
| title | Probing patchy reionisation with JWST: IGM opacity constraints from the Lyman-$α$ forest of galaxies in legacy extragalactic fields |
| topic | Astrophysics of Galaxies Cosmology and Nongalactic Astrophysics |
| url | https://arxiv.org/abs/2504.02683 |