We report new spectroscopy of two star-forming galaxies with strong Lyα emission at z = 6.03 and 6.04 in the Subaru Deep Field. These two objects are originally selected as i′-dropouts (i′ -z′ > 1.5) showing an interesting photometric property, the "NB921 depression." The NB921-band (centered at 9196 Å) magnitude is significantly depressed with respect to the z′-band magnitude. The optical spectra of these two objects exhibit asymmetric emission lines at λobs ∼ 8540 and ∼8560 Å, suggesting that these objects are Lyα emitters at z ∼ 6. The rest-frame equivalent widths of the Lyα emission of the two objects are 94 and 236 Å; the latter is the Lyα emitter with the largest Lyα equivalent width at z ≳ 6 ever spectroscopically confirmed. The spectroscopically measured Lyα fluxes of these two objects are consistent with the interpretation that the NB921 depression is caused by the contribution of the strong Lyα emission to the z′-band flux. Most of the NB921-depressed i′-dropout objects are thought to be strong Lyα emitters at 6.0 ≲ z ≲ 6.5; Galactic L and T dwarfs and NB921-dropout galaxies at z > 6.6 do not dominate the NB921-depressed i′-dropout sample. Thus, the NB921 -depression method is very useful for finding high-z Lyα emitters with a large Lyα equivalent width over a large redshift range, 6.0 ≲ z ≲ 6.5. Although the broadband-selected sample at z ∼ 3 contains only a small fraction of objects with a Lyα equivalent width larger than 100 Å, the i′-dropout sample of the Subaru Deep Field contains a much larger fraction of such strong Lyα emitters. This may imply a strong evolution of the Lyα equivalent width from z ≳ 6 to z ∼ 3. © 2005. The American Astronomical Society. All rights reserved.

We report new follow-up spectroscopy of i′-dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed 1′-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 153 Å and 114 Å, 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 100 Å 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 100 Å is significantly higher at z ∼ 6 (the cosmic age of ∼1 Gyr) than that at z ∼ 3 (∼2 Gyr), 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. © ESO 2007.