A faint M$_{\rm UV} = -14.5$ Lyman continuum leaker in the reionization epoch: unprecedented Ly$α$ properties at z=5.725

TL;DR

The paper investigates AMORE6, an extremely metal-poor and ultra-compact dwarf galaxy at $z \approx 5.725$ magnified by lensing, as a prime LyC-leaking candidate during the reionization epoch. Through multi-instrument spectroscopy, it reveals a near-systemic, narrow Ly$\alpha$ emission with $\mathrm{EW}_{Ly\alpha} = 150 \pm 10$ Å and $\mathrm{FWHM}_{Ly\alpha} = 58 \pm 1$ km s$^{-1}$, with a negligible velocity offset $\Delta v = 4 \pm 67$ km s$^{-1}$. Radiative-transfer modelling yields two viable Ly$\alpha$-emitting scenarios, but the data favor an extremely low HI column density with $\log N_{HI} \lesssim 14$, implying a high LyC escape fraction. This makes AMORE6 one of the most compelling LyC leakers found in the reionization era and underscores the potential of Ly$\alpha$ near-systemic profiles as indirect diagnostics for LyC leakage, motivating future JWST/NIRSpec observations to solidify systemic redshift, kinematics, and line inventory.

Abstract

We report the unprecedented Ly$α$ properties of AMORE6, an extremely metal-poor ($12+\log({\rm O/H}) < 6$), low-mass ($M_\star = 4.4\times10^{5}\,M_\odot$), and ultra-compact (effective radius $\lesssim30$ pc) dwarf galaxy at $z=5.7253$, gravitationally lensed by the cluster A2744. A prominent, narrow, and nearly-symmetric Ly$α$ emission line is detected at the systemic redshift (the latter traced by H$β$, from JWST/NIRCam slitless spectroscopy), with rest-frame $EW=150\pm10$ Å, $\rm FWHM=58\pm1$ km s$^{-1}$, and a slight asymmetry, resulting in a $\rm \sim10\%$ flux excess in the red wing of the line. The negligible velocity offset from systemic ($dv = 4\pm67$ km s$^{-1}$, $3σ$ uncertainty), together with the sharpness and symmetry of the profile, indicates minimal radiative transfer effects implying a neutral hydrogen column density consistent with an optically thin medium, compatible with a non-zero ionizing photon escape fraction. If indirect spectral diagnostics calibrated at $z<4.5$ remain the only viable tools to identify LyC leakers during reionization, then based on its strongest indicator (Ly$α$), AMORE6 stands out as one of the most compelling LyC-leaking candidates yet discovered in the reionization epoch.

Figures (4)

• Figure 1: (Left): NIRCam stacking of the SW bands, with the red contours highlighting the extent of the α Ly$\alpha$ emission. (Middle): α Ly$\alpha$ flux from VLT/MUSE; a faint emission from a low-magnification third counter-image (AMORE6-C) is also visible. (Right): Zoomed two-dimensional X-Shooter spectrum centered on the α Ly$\alpha$ wavelength, from 8163 Å (left)) to 8184 Å (right). The short-dashed lines show the region used for spectral extraction.
• Figure 2: Observed profile of the α Ly$\alpha$ emission for AMORE6-B (black line), best-fit profile (red solid), best-fit central wavelength (red dashed), systemic redshift (and relative uncertainty) from Morishita2025 (blue solid line and shaded region). The line flux within $\rm \pm100~km~s^{-1}$ of the systemic redshift ($\rm f_{cen}$, see text) is marked as a gray shaded region. The (lower-resolution) spectrum of the same source from MUSE is shown as black dashed line.
• Figure 3: Comparison of the α Ly$\alpha$ properties of AMORE6 with literature samples of galaxies at $\rm z>4$ (including lensed galaxies). Left panel: shift of the α Ly$\alpha$ peak with respect to systemic redshift ($dv$) in function of the rest-frame α Ly$\alpha$ equivalent width; the literature samples are taken from Tang2024Saxena2024PrietoLyon2025_arXiv; for the references of the lensed galaxies sample see Appendix \ref{['sec:app:lensed_sample']}. Central panel: EW(α Ly$\alpha$) in function of $\rm M_{UV}$; AMORE6 is 4-5 magnitudes fainter than the average galaxies we are comparing with. Right panel:$dv$ in function of $\rm FWHM_{Ly\alpha}$, for the fewer cases where data for both quantities are available; PrietoLyon2025_arXiv was the first work providing a statistical sample of α Ly$\alpha$ FWHM measurements in $\rm z\sim5-6$ galaxies.
• Figure 4: Emergent α Ly$\alpha$ line profiles from a grid of idealized radiative-transfer models (see text). Left: high-velocity shell (V$_{\rm max} = 750$ km s$^{-1}$); varying $\rm N_{HI}$ shows that $\rm log(N_{HI})\lesssim20$ reproduces the observed profile but requires an extreme outflow. Right: low-velocity case (V$_{\rm max} = 100$ km s$^{-1}$); matching the data demands very low columns, $\rm log(N_{HI})\lesssim14$. Both panels assume the systemic redshift $z=5.7253$.