Resolving dust and Lyman α emission in a lensed galaxy at the epoch of reionization with JWST/CANUCS

¹ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, SI-1000 Ljubljana, Slovenia
² Department of Physics and Astronomy, University of California Davis, 1 Shields Avenue, Davis, CA 95616, USA
³ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
⁴ Beus Center for Cosmic Foundations, Arizona State University, Tempe, AZ 85287, USA
⁵ Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV, Groningen, The Netherlands
⁶ INAF – Osservatorio Astronomico di Roma, Via Frascati 33. Monte Porzio Catone, 00078, Italy
⁷ Department of Astronomy and Physics and Institute for Computational Astrophysics, Saint Mary’s University, 923 Robie Street, Halifax, Nova Scotia, B3H 3C3, Canada
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA
⁹ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
¹⁰ Dunlap Institute for Astronomy and Astrophysics, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹¹ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
¹² Cosmic Dawn Center (DAWN), Denmark
¹³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen N, Denmark
¹⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
¹⁵ Department of Physics and Astronomy, York University, 4700 Keele St. Toronto, Ontario M3J 1P3, Canada
¹⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁷ Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
¹⁸ Cavendish Laboratory – Astrophysics Group, University of Cambridge, 19 JJ Thomson Avenue, Cambridge CB3 0HE, UK

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Received: 15 December 2025
Accepted: 25 February 2026

Abstract

Context. Lyman α (Lyα) emission is highly sensitive to dust and neutral hydrogen and is believed to be suppressed in dusty or hydrogen-rich galaxies – especially during the epoch of reionization (EoR). Yet, moderately dusty Lyα emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows facilitate Lyα escape.

Aims. We investigated the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at z = 6.5676 to characterize its multiphase ISM structure and the physical conditions that regulate Lyα escape.

Methods. We combined JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES spectral-energy-distribution-fitting framework with a flexible attenuation law, we derived spatially resolved stellar, nebular, and dust properties on integrated (≈1 kpc), slit-level (≈0.1 kpc), and pixel-level (≈25 pc) measurements, thanks to strong lensing with a magnification of μ ≈ 8.3 − 9.1. A high-quality Lyα map from SLEUTH, a tool for extracting spatially resolved emission-line maps from slitless spectroscopy, traces the spatial distribution of Lyα emission.

Results. We measure an unlensed stellar mass of log M_* = 8.3 − 8.4 and an intrinsic UV magnitude of M_UV = −19.8 ± 0.1. Slit-level measurements show that the oldest, most massive region (S1) is moderately dusty with consistent stellar (A_V, β) and nebular (A_V^B, line-emission) indicators, implying a uniform ISM geometry, while the dust tracers of the youngest region (S3) are highly mismatched, revealing a complex, feedback-shaped multiphase ISM. Lyα emission arises primarily from S3. Pixel-level maps reveal a dust-cleared central clump (C3) where Lyα emerges, encircled by dustier outskirts, consistent with a very recent (≲10 Myr) starburst that created Lyα escape channels. Next, slit-level maps show Calzetti-like attenuation curves with a UV bump that increases with stellar age and decreases with V-band attenuation (A_V), with a tentative detection of a UV bump in S1 at ∼25% of the Milky Way amplitude. Pixel-level maps reveal that both the curve slope (S) and the UV bump (B) peak in the region between two clumps (C1 and C2), indicating dust-grain processing in a merger-driven starburst.

Conclusions. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE at the EoR, demonstrating how the interplay between multiphase ISM geometry and feedback governs Lyα escape. Constraining some key quantities requires fully resolved spectroscopy, which future JWST/NIRSpec integral field unit observations will provide.