A faint M${}_{\mathrm{UV}}=-14.5$ Lyman-continuum leaker candidate in the epoch of reionization: Unprecedented Ly$\hat{I}\pm$ properties at z=5.725

Abstract

We report the unprecedented Ly$\hat{I}\pm$ properties of AMORE6, an extremely metal-poor ($12+\log (O/H)<6$), low-mass ($M_{\star }=4.4\times 10^5{M}_{\odot }$), and ultracompact (effective radius $\sim 30$ pc) dwarf galaxy at $z=5.7253$, which is gravitationally lensed by the cluster A2744. A prominent, narrow, and nearly symmetric Ly$\hat{I}\pm$ emission line is detected at the systemic redshift (the latter traced by H${\hat{I}}^2$, from JWST/NIRCam slitless spectroscopy), with a rest-frame equivalent width of $150\pm 10$ à , a full width at half maximum of $58\pm 1$ km s${}^{-1}$, and a slight asymmetry, resulting in a flux excess of $\sim 10\%$ in the red wing of the line. The negligible velocity offset from systemic ($dv=4\pm 67$ km s${}^{-1}$, $3\ddot{I}ƒ$ 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 (Ly$\hat{I}\pm$), AMORE6 stands out as one of the most compelling LyC-leaking candidates yet discovered in the epoch of reionization.

6 pages, 4 figures, 2 tables (including appendices). Letter accepted for publication in A&A

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