Letter to the Editor

A faint M_UV = −14.5 Lyman-continuum leaker candidate in the epoch of reionization: Unprecedented Lyα properties at z = 5.725

¹ INAF – OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129, Bologna, Italy
² IPAC, California Institute of Technology, Pasadena, CA 91125, USA
³ Astronomical Institute, Tohoku University, 6-3 Aramaki, Aoba-ku, Sendai, 980-8578, Japan
⁴ Space Telescope Science Institute (STScI), 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵ Department of Physics and Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA
⁶ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139, USA
⁷ Astronomical Institute, Graduate School of Science, Tohoku University, Sendai, Miyagi, 980-8578, Japan
⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, D-85748, Garching, Germany
⁹ Univ Lyon, Univ Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, Saint-Genis-Laval, France
¹⁰ Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290, Versoix, Switzerland
¹¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, I-20133, Milano, Italy
¹² INAF – IASF Milano, Via A. Corti 12, I-20133, Milano, Italy
¹³ Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, I-44122, Ferrara, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 December 2025
Accepted: 27 February 2026

Abstract

We report the unprecedented Lyα properties of AMORE6, an extremely metal-poor (12 + log(O/H) < 6), low-mass (M_★ = 4.4 × 10⁵ M_⊙), and ultracompact (effective radius ∼30 pc) dwarf galaxy at z = 5.7253, which is gravitationally lensed by the cluster A2744. A prominent, narrow, and nearly symmetric Lyα emission line is detected at the systemic redshift (the latter traced by Hβ, from JWST/NIRCam slitless spectroscopy), with a rest-frame equivalent width of 150 ± 10 Å, a full width at half maximum of 58 ± 1 km s⁻¹, and a slight asymmetry, resulting in a flux excess of ∼10% in the red wing of the line. The negligible velocity offset from systemic (dv = 4 ± 67 km s⁻¹, 3σ uncertainty), together with the sharpness and symmetry of the profile, indicates minimum radiative transfer effects, which implies a neutral hydrogen column density consistent with an optically thin medium that in turn is compatible with a nonzero ionizing photon escape fraction. If indirect spectral diagnostics calibrated at z < 4.5 remain the only viable tools for identifying LyC leakers during reionization, then based on its strongest indicator (Lyα), AMORE6 stands out as one of the most compelling LyC-leaking candidates yet discovered in the epoch of reionization.