  Using Ly{\alpha} emission line as a tracer of high redshift star forming
galaxies, hundreds of Ly{\alpha} emission line galaxies (LAEs) at z > 5 have
been detected. These LAEs are considered to be low mass young galaxies,
critical to the reionization of the universe and the metal enrichment of
circumgalactic medium (CGM) and intergalactic medium (IGM). It is assumed that
outflows in LAEs can help ionizing photons and Ly{\alpha} photons escape out of
galaxies. However we still know little about the outflows in high redshifts
LAEs due to observational difficulties, especially at redshift > 5. Models of
Ly{\alpha} radiative transfer predict asymmetric Ly{\alpha} line profiles with
broad red wing in LAEs with outflows. Here we report a z ~ 5.7 Ly{\alpha}
emission line with a broad red wing extending to > 1000 km/s relative to the
peak of Ly{\alpha} line, which has been detected in only a couple of z > 5 LAEs
till now. If the broad red wing is ascribed to gas outflow instead of AGN
activity, the outflow velocity could be larger than the escape velocity (~ 500
km/s) of typical halo mass of z ~ 5.7 LAEs, being consistent with the picture
that outflows in LAEs disperse metals to CGM and IGM.
