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

TL;DR

This work presents a spatially resolved study of the moderately dusty Ly$\ alpha$ emitter HCM 6A at $z=6.5676$, exploiting JWST CANUCS data and strong gravitational lensing ($\mu\approx8.3$–$9.1$) to map dust, gas, and stellar properties from ~kpc down to ~25 pc scales. By applying a flexible BAGPIPES SED-fitting framework to global, slit-level, and pixel-level data, the authors uncover a multiphase ISM where feedback-driven clearing creates Ly$\ alpha$ escape channels, especially in the dust-cleared core of the youngest clump C3. They derive Calzetti-like attenuation curves with a UV bump that strengthens with stellar age and decreasing $A_V$, and demonstrate that Ly$\ alpha$ emission preferentially originates from cleared regions despite the presence of dust. Overall, the results highlight how small-scale ISM geometry and feedback regulate Ly$\ alpha$ escape in the EoR and showcase the power of spatially resolved JWST spectroscopy for probing early galaxy evolution.

Abstract

Lyman $α$ emission is highly sensitive to dust and neutral hydrogen and is therefore expected to be strongly suppressed in dusty or gas-rich galaxies during the epoch of reionization (EoR). Nevertheless, numerous moderately dusty Ly$α$ emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows may facilitate Ly$α$ escape. We investigate the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at $z=6.5676$ to characterize its multiphase ISM and the physical conditions regulating Ly$α$ escape. We combine JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES SED-fitting framework with a flexible attenuation law, we derive stellar, nebular, and dust properties on integrated ($\sim$kpc), slit-level ($\sim$0.1 kpc), and pixel-level ($\sim$25 pc) scales, enabled by strong lensing ($μ\approx 8.3$-$9.1$). A Ly$α$ map from SLEUTH traces the spatial distribution of Ly$α$ emission. We measure an unlensed stellar mass of $\log M_\ast = 8.3$-$8.4$ and an intrinsic UV magnitude of $M_{\rm UV} = -19.8 \pm 0.1$. The older region (S1) is moderately dusty with consistent stellar and nebular attenuation indicators, implying a uniform ISM geometry, while the youngest region (S3) shows strong discrepancies among dust tracers, indicating a feedback-shaped multiphase ISM. Ly$α$ emission arises primarily from S3, where a dust-cleared central clump enables efficient Ly$α$ escape. We find Calzetti-like attenuation curves with a UV bump that strengthens with stellar age and decreasing $A_V$. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE during the EoR, demonstrating how feedback and multiphase ISM structure govern Ly$α$ escape.

Figures (5)

• Figure 1: NIRCam imaging, NIRSpec/PRISM spectroscopy, and the spectro-photometric fit for the bright LAE HCM 6A at $z = 6.5676$. Top left: RGB composite image of the system constructed from JWST/NIRCam 20 mas imaging by assigning F200W to the red channel, F150W to the green channel, and F115W to the blue channel. Middle left: NIRCam F115W image of the system. The three adjacent NIRSpec slits (NIRSpec IDs: 2101494, 2190001, and 2190002; S1, S2, and S3 hereafter) are overlaid on the RGB and F115W images targeting the three main clumps C1-C3. Clumps C1 and C3 exhibit internal substructure and are composed of mini-clumps (C1a-C1b and C3a–C3e, respectively), ordered from brightest to faintest in the RGB image. Top right and bottom panels: Slit-level photometry (teal circles), photometry-rescaled NIRSpec PRISM slit spectroscopy (black line), and the slit-level spectro-photometric fit using customized BAGPIPES (red line). The vertical dashed lines indicate the wavelengths of key UV and optical emission lines (all detected except for Ly$\alpha$). The vertical orange strip indicates the wavelength range of the UV bump absorption feature. The reduced chi-square, $\chi^2_\nu$, for each fit is shown in the top right. Inset panels: Basic galaxy properties derived from the SED fitting.
• Figure 2: NIRISS 1D slitless spectrum of HCM 6A, showing a clear detection of the Ly$\alpha$ emission line at $\lambda_{\rm obs} \approx 9200 \AA$, consistent with $z \approx 6.6$. The spectrum combines extractions from the NIRISS GR150C and GR150R grisms crossed with the F090WN, F115WN, F150WN, and F200WN filters. The vertical dashed line indicates the observed wavelength of the detected Ly$\alpha$ emission line.
• Figure 3: Slit-level, spatially resolved physical properties of the target system: stellar mass ($\log M_*$; a), star formation rate (SFR; b), mass-weighted stellar age (${\langle a \rangle}_*^{\mathrm{m}}$; c), ionization parameter ($\log U$; d), $V$-band attenuation ($A_V$; e), slope ($S$; f) and UV bump ($B$; g) of the dust attenuation curve, attenuation derived from the Balmer decrement ($A_V^{\mathrm{B}}$; h), and UV continuum slope ($\beta$; i). Both $\log M_*$ and SFR are corrected for the lensing magnification. The NIRCam 20mas F115W map of the system is shown as the background image.
• Figure 4: Pixel-level, spatially resolved physical properties of the target system: stellar mass ($\log M_*$; a), star formation rate (SFR; b), mass-weighted stellar age (${\langle a \rangle}_*^{\mathrm{m}}$; c), ionization parameter ($\log U$; d), $V$-band attenuation ($A_V$; e), slope ($S$; f) and UV bump ($B$; g) of the dust attenuation curve, sSFR (h), and Ly$\alpha$ flux ($f_{\mathrm{Ly\alpha}}$; i). The quantities $\log M_*$, SFR, and $f_{\mathrm{Ly\alpha}}$ are corrected for lensing magnification. The three adjacent NIRSpec slits are overlaid as cyan rectangles. The NIRCam RGB map is shown as white contours at the 5$\sigma$ and 10$\sigma$ levels, while the Ly$\alpha$ emission is traced by magenta contours at 1-2$\sigma$.
• Figure 5: Dust attenuation curves of the three slit regions at $z = 6.5676$ from the slit-level spectro-photometric fits. Inset panel: Star formation histories (SFHs).