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We report the discovery of a double-peaked Lyman-$α$ (Ly$α$) emitter (LAE) at z=3.2177±0.0001 in VLT/MUSE data. The galaxy is strongly lensed by the galaxy cluster RXC~J0018.5+1626 recently observed in the RELICS survey, and the double-peaked Ly$α$ emission is clearly detected in the two counter images in the MUSE field-of-view. We measure a relatively high Ly$α$ rest-frame equivalent width (EW) of EWLyα,0=(63±2)\,A. Additional near-infrared (NIR) spectroscopy allows us to measure the H$β$, [OIII]λ4959\,A and [OIII]λ5007\,A emission lines, which show moderate rest-frame EWs of the order of a few ~10-100\,A, an [OIII]λ5007\,A/H$β$ ratio of 4.8±0.7, and a lower limit on the [OIII]/[OII] ratio of $>5.6$. The galaxy has very blue UV-continuum slopes of βFUV=-2.23±0.06 and βNUV=-3.0±0.2, and is magnified by factors μ~7-10 in each of the two images, thus enabling a view into a low-mass (M_107.5\,M_) high-redshift galaxy analog. Notably, the blue peak of the Ly$α$ profile is significantly stronger than the red peak, which suggests an inflow of matter and possibly very low HI column densities in its circumgalactic gas. Combined with the high lensing magnification and image multiplicity, these properties make this galaxy a prime candidate for follow-up observations to search for LyC emission and constrain the LyC photon escape fraction.
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Furtak et al. (2022) studied this question.