2007A&A...468..877N

We report new follow-up spectroscopy of i'-dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed i'-dropout selection method is expected to select galaxies with large equivalent-width Lyα emission over a wide redshift range, 6.0≲z≲6.5. Two of four observed targets show a strong emission line with a clear asymmetric profile, identified as Lyα emitters at z=6.11 and 6.00. Their rest-frame equivalent widths are 153Å and 114Å, which are lower limits on the intrinsic equivalent widths. Through our spectroscopic observations (including previous ones) of NB921-depressed i'-dropout galaxies, we identified 5 galaxies in total with a rest-frame equivalent width larger than 100Å at 6.0≲z≲6.5 out of 8 photometric candidates, which suggests that the NB921-depressed i'-dropout selection method is possibly an efficient way to search for Lyα emitters with a large Lyα equivalent width, in a wider redshift range than with usual narrow-band excess techniques. By combining these findings with our previous observational results, we infer that the fraction of broad-band selected galaxies having a rest-frame equivalent width larger than 100Å is significantly higher at z∼6 (the cosmic age of ∼1Gyr) than that at z∼3 (∼2Gyr), being consistent with the idea that the typical stellar population of galaxies is significantly younger at z∼6 than that at z∼3. The NB921-depressed i'-dropout galaxies may be interesting candidates for hosts of massive, zero-metallicity Population III stars.