Using the Lyα emission line as a tracer of high-redshift, star-forming galaxies, hundreds of Lyα emission line galaxies (LAEs) at z 5 have been detected. These LAEs are considered to be low-mass young galaxies, critical to the re-ionization of the universe and the metal enrichment of the circumgalactic medium (CGM) and the intergalactic medium (IGM). It is assumed that outflows in LAEs can help both ionizing photons and Lyα photons escape from galaxies. However, we still know little about the outflows in high-redshift LAEs due to observational difficulties, especially at redshift 5. Models of Lyα radiative transfer predict asymmetric Lyα line profiles with broad red wings in LAEs with outflows. Here, we report a z ~ 5.7 Lyα emission line with a broad red wing extending to 1000?km s–1 relative to the peak of Lyα line, which has been detected in only a couple of z 5 LAEs until now. If the broad red wing is ascribed to gas outflow instead of active galactic nucleus activity, the outflow velocity could be larger than the escape velocity (~500?km s–1) of a typical halo mass of z ~ 5.7 LAEs, which is consistent with the idea that outflows in LAEs disperse metals to CGM and IGM.
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