AN ULTRALUMINOUS LYα EMITTER WITH A BLUE WING AT z = 6.6* **

We report the detection of the most luminous high-redshift Lyα emitting galaxy (LAE) yet seen, with . The galaxy—COSMOS Lyα1, or COLA1—was detected in a search for ultraluminous LAEs with Hyper Suprime-Cam on the Subaru telescope. It was confirmed as lying at z = 6.593, based on a Lyα line detection obtained from follow-up spectroscopy with the DEIMOS spectrograph on Keck II. COLA1 is the first very high-redshift LAE to show a multi-component Lyα line profile with a blue wing, which suggests that it could lie in a highly ionized region of the intergalactic medium (IGM) and could have significant infall. If this interpretation is correct, then ultraluminous LAEs like COLA1 offer a unique opportunity to determine the properties of the H ii regions around these galaxies, which will help in understanding the ionization of the z ~ 7 IGM.