An OASIS of Lyman-$α$ within a neutral intergalactic desert: reaffirmed line and blue continuum reveal efficient ionising agents at $z = 13$

Abstract

$\require{mediawiki-texvc}$Galaxy assembly was already well underway in the first 400 Myr of cosmic time, as recently revealed by JWST. However, the contribution of these early galaxies to cosmic reionisation remains uncertain. Here we present new JWST/NIRSpec observations of GS-z13-1-LA obtained as part of the OASIS and JADES programmes, whose combined deep (56 h) NIRSpec/PRISM spectrum confirms the Lyman-$\mathrmα$ line detection and blue UV continuum at redshift $z = 13.1$ presented in a previous work. The measured Lyman-$\mathrmα$ emission (rest-frame equivalent width of $66_{-9}^{+10}\,Å$) and steep continuum slope ($β_\text{UV} \approx -3$) point towards GS-z13-1-LA hosting a remarkably hot and powerful ionising source, and allow at most a modest contribution from the nebular continuum. The steep turnover of the continuum is still present, but less pronounced in the new OASIS spectrum. Combined, this implies that ionising photons may escape GS-z13-1-LA at a sufficient rate to weaken the other, still undetected UV lines, and to lead the formation of a small ionised bubble ($R_\text{ion} \approx 0.2\,\mathrm{pMpc}$). A yet larger bubble could alleviate the required ionising production efficiency of GS-z13-1-LA from $ξ_\mathrm{ion} \approx 10^{26.4}\,\mathrm{Hz\,erg^{-1}}$ down to $\approx 10^{25.9}\,\mathrm{Hz\,erg^{-1}}$, still extremely high but more readily reconcilable with stellar models. In turn, this would require a notable overdensity of galaxies with highly efficient ionising capabilities, a scenario for which tentative evidence is found in the form of 16 nearby photometric candidates and one spectroscopically confirmed source, JADES-GS-z13-0. The new OASIS observations therefore confirm the overall picture of GS-z13-1-LA as an early beacon of reionisation, providing compelling evidence for its start only 330 Myr after the Big Bang.

Figures (7)

• Figure 1: NIRCam and NIRSpec/PRISM observations of GS-z13-1-LA. a, False-colour NIRCam image with the F090W, F115W, F150W, and F162M filters shown as blue, F182M, F200W, F210M, F250M, and F277W as green, and F300M, F335M, F356W, F410M, F444W, and F480M as red (stacked at native resolution). For reference, the point spread function FWHM of the F115W, F200W, and F356W filters are shown in the bottom-left corner. The triplet of NIRSpec micro-shutters that were opened in the JADES and OASIS observations are shown in teal and orange, respectively. A physical scale of $1 \, \mathrm{kpc}$ at $z = 13$ is indicated by the horizontal white line. b, One-dimensional NIRSpec/PRISM spectrum from JADES 2025Natur.639..897W. c, PRISM spectra from the completed JADES (teal) and new OASIS (orange) observations, and the combined spectrum (dark blue). The grey shading indicates the wavelength range over which the Ly$\upalpha$ line fluxes are integrated (\ref{['tab:Lya_fluxes']}). $1 \sigma$ uncertainty on individual wavelength bins is shown by dashed lines in panels b and c.
• Figure 2: Modelled NIRSpec/PRISM observations of GS-z13-1-LA. a, Model curves for the IGM transmission $T$, according to the legend on the right. b, Coloured lines show the path-loss corrected PRISM spectra observed in JADES and OASIS (\ref{['ssec:JWST_observing_programmes']}). The best-fit model spectrum is shown by the black solid line, while its individual power-law and nebular-continuum components are shown with dashed and dotted lines, respectively. The observed Ly$\upalpha$ flux and EW in the best-fit model are annotated. The expected location of rest-frame UV lines is indicated for the model's best-fit systemic redshift of $z = 13.07$, although we note that none are confidently detected (\ref{['sec:Discussion']}). c, Uncertainty-normalised residuals $\chi$ are shown for the two different spectra.
• Figure 3: a, Coloured lines show blackbody curves of varying temperature (according to the colourbar in panel b). Thin dashed, dash-dotted, and dotted lines show nebular continuum predictions by pyneb2015AA...573A..42L for varying electron density $n_e$, but at fixed temperature $T_e = 10000.0 \, \mathrm{K}$ (where they are bluest; cf. panel b). Grey shading indicates the approximate wavelength range probed in the spectral modelling of GS-z13-1-LA (\ref{['sssec:Spectral_modelling_setup']}). b, Effective UV slopes of blackbody spectra and nebular continua measured across the wavelength range highlighted in panel a, as a function of temperature. Thin lines show the minimum slope of nebular continua at different densities (according to the legend in panel a). A horizontal black line and grey shading shows the inferred UV slope of GS-z13-1-LA for a pure power-law (PL) continuum, while the horizontal dashed line indicates the upper limit obtained for the intrinsic power-law slope when including nebular continuum. Grey shading similarly indicates the inferred range on the top $\xi_\text{ion}$ axis. The horizontal dotted line shows the blackbody spectral slope in the Rayleigh-Jeans limit, $F_\lambda \propto \lambda^{-4}$.
• Figure 4: a, Histogram of photometric redshifts from 2026arXiv260115959H. Members of the candidate overdensity identified by 2026arXiv260115960W are highlighted in green, those found in the vicinity of GS-z13-1-LA in blue. Two $z \approx 15$ candidates enter the selection, since accounting for DLA absorption significantly decreases their estimated redshift (see text for details). b, Spatial distribution of galaxy candidates found within the JADES NIRCam footprint (solid green line; cf. \ref{['ssec:JWST_observing_programmes']}) at $12 < z_\text{phot} < 14$, which are shown as small, open black hexagons. Larger hexagons mark the positions of JADES-GS-z13-0 2023NatAs...7..611R2023NatAs...7..622C2024ApJ...976..160H, GS-z13-1-LA (both filled black and annotated), and $16$ nearby galaxy candidates. The solid and dashed grey circles around GS-z13-1-LA show the projected sizes of the ionised regions in our single-source and collective model, $R_\text{ion} \approx 0.2 \, \mathrm{pMpc}$ or $0.5 \, \mathrm{pMpc}$, respectively (\ref{['sssec:Spectral_modelling_results']}). c, NIRCam photometry of GS-z13-1-LA (black hexagons) and nearby candidates (open circles). A vertical dashed line indicates the observed Ly$\upalpha$ wavelength in GS-z13-1-LA, which falls close to the F182M medium-band filter where a tentative flux excess is seen in the brightest candidate (ID 45716, highlighted in blue).
• Figure 5: Two-dimensional spectra from JADES and OASIS, both in the default reduction where all three nod positions are considered ('3-nod'), and in the case where only the two outer nod positions are considered ('2-nod', leading to reduced SNR but avoiding potential self-subtraction). The line and continuum emission is observed to be spatially compact. The bottom panel further shows little difference between one-dimensional spectra extracted in the two reductions, suggesting that the effect of self-subtraction is limited.