2005ApJ...631L...5N

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 He II λ1640. 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 σ upper limit of 9.06x10^–18 ergs/s/cm2 on the He II λ1640 emission-line flux, which corresponds to a luminosity of 4.11x10⁴² ergs/s. This upper limit on the He II λ1640 luminosity implies that the upper limit on the Population III star formation rate is in the range 4.9-41.2 M_☉/yr if Population III stars suffer no mass loss and in the range 1.8-13.2 M_☉/yr 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.