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 Lyα emitter with only very faint ultraviolet continuum. The rest-frame equivalent width of the detected Lyα emission is as much as 130 Å. Thus the light detected in our z′ image is largely attributed to the Lyα emission, i.e., ~40% of the z′-band flux is the strong Lyα 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.