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 Ly alpha at z = 6.33, SDF J132440.6+273607, which we used to search for He II lambda 1640. This emission line is expected if the target hosts a significant number of Population III stars. Even after 42 ks of integration with the Subaru/OHS spectrograph, no emission-line features are detected in the J and H bands, which confirms that SDF J132440.6+273607 is neither an active galactic nucleus nor a low-z emission-line object. We obtained a 2 sigma upper limit of 9.06 x 10(-18) ergs s(-1) cm(-2) on the He II lambda 1640 emission-line flux, which corresponds to a luminosity of ergs s(-1). This upper limit on the He II lambda 1640 luminosity implies that the upper limit on the Population III star formation rate is in the range 4.9-41.2 M-circle dot yr(-1) if Population III stars suffer no mass loss and in the range 1.8-13.2 M-circle dot yr(-1) if strong mass loss is present. The nondetection of He II in SDF J132440.6+273607 at z = 6.33 may thus disfavor weak feedback models for Population III stars.