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

We present a very deep near-infrared spectroscopic observation of a strongLy_alpha emitter at z=6.33, SDF J132440.6+273607, which we used to search forHeII 1640. This emission line is expected if the target hosts a significantnumber of population III stars. Even after 42 ksec of integration with theSubaru/OHS spectrograph, no emission-line features are detected in the JH band,which confirms that SDF J132440.6+273607 is neither an active galactic nucleusnor a low-$z$ emission-line object. We obtained a 2sigma upper-limit of9.06e-18 ergs/s/cm^2 on the HeII 1640 emission line flux, which corresponds toa luminosity of 4.11e42 ergs/s. This upper-limit on the HeII 1640 luminosityimplies that the upper limit on population III star-formation rate is in therange 4.9--41.2 M_sun/yr if population III stars suffer no mass loss, and inthe range 1.8--13.2 M_sun/yr if strong mass loss is present. The non-detectionof HeII in SDF J132440.6+273607 at z=6.33 may thus disfavor weak feedbackmodels for population III stars.Comment: 13 pages, 3 figures, to appear in The Astrophysical Journal Letter