An Observational Pursuit for Population III Stars in a Lyα Emitter at z=6.33 through Heii Emission

We present a very deep near-infrared spectroscopic observation of a strong Lyα emitter at z = 6.33, SDF J132440.6+273607, which we used to search for Heiiλ1640. This emission line is expected if the target hosts a significant number of population III stars. Even after 42 ksec of integration with the Subaru/OHS spectrograph, no emission-line features are detected in the JH band, which confirms that SDF J132440.6+273607 is neither an active galactic nucleus nor a low-z emission-line object. We obtained a 2σ upper-limit of 9.06×10 ergs s cm on the Heiiλ1640 emission line flux, which corresponds to a luminosity of 4.11×10 ergs s. This upper-limit on the Heiiλ1640 luminosity implies that the upper limit on population III star-formation rate is in the range 4.9−41.2M⊙yr −1 if population III stars suffer no mass loss, and in the range 1.8− 13.2M⊙yr −1 if strong mass loss is present. The non-detection of Heii in SDF J132440.6+273607 at z = 6.33 may thus disfavor weak feedback models for population III stars. Subject headings: early universe — galaxies: evolution — galaxies: formation — galaxies: individual (SDF J132440.6+273607) — galaxies: starburst — stars: early-type Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Institute of Astronomy, Graduate School of Science, University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan Astronomical Institute, Graduate School of Science, Tohoku University, Aramaki, Aoba, Sendai 9808578, Japan Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A’ohoku Place, Hilo, HI 96720