A strong Lyα emitter at z=6.33 in the Subaru Deep Field selected as an i′-dropout

We report on the discovery of a star-forming galaxy at z = 6.33 in the Subaru Deep Field. This object is selected as a candidate of an i'-dropout high-redshift galaxy around z = 6 because of its red i' - z' color in our deep optical imaging survey in the Subaru Deep Field. Our follow-up optical spectroscopy reveals that this object is a strong Lyalpha emitter with only very faint ultraviolet continuum. The rest-frame equivalent width of the detected Lyalpha emission is as much as 130 Angstrom. Thus the light detected in our z' image is largely attributed to the Lyalpha emission, i.e., similar to40% of the z'-band flux is the strong Lyalpha emission, giving a very red i' - z' color. This is consistent with the photometric property of this object because the narrowband data obtained with the NB921 filter show a significant depression, z' - NB921 = -0.54 mag. By using the photometric data, we show that some other objects among the 48 i'-dropout high-redshift galaxy candidates found in the Subaru Deep Field also show a significant NB921 depression. We briefly discuss the nature of these NB921-depressed objects.