High-redshift Ly emitters with a large equivalent width

Properties of i-dropout galaxies with an NB921-band depression in the Subaru deep field

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: tohru@arcetri.astro.it
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 151-8588, Japan
³ Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan
⁴ INAF – Osservatorio Astrofisico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
⁶ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA
⁷ Department of Astronomy, University of California at Los Angeles, PO Box 951547, Los Angeles, CA 90095-1547, USA
⁸ Institute of Astronomy, Graduate School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁹ Department of Astronomy, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-8502, Japan
¹⁰ Department of Physics, Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan

Received: 3 August 2006
Accepted: 12 January 2007

Abstract

We report new follow-up spectroscopy of -dropout galaxies with an NB921-band depression found in the Subaru Deep Field. The NB921-depressed -dropout selection method is expected to select galaxies with large equivalent-width Lyα emission over a wide redshift range, . 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  and 114 , which are lower limits on the intrinsic equivalent widths. Through our spectroscopic observations (including previous ones) of NB921-depressed -dropout galaxies, we identified 5 galaxies in total with a rest-frame equivalent width larger than 100  at  out of 8 photometric candidates, which suggests that the NB921-depressed -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  is significantly higher at (the cosmic age of ~1 Gyr) than that at (~2 Gyr), being consistent with the idea that the typical stellar population of galaxies is significantly younger at than that at . The NB921-depressed -dropout galaxies may be interesting candidates for hosts of massive, zero-metallicity Population III stars.