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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 astrong Lyα emitter at z=6.33, SDF J132440.6+273607, which we usedto search for He II λ1640. This emission line is expected if thetarget hosts a significant number of Population III stars. Even after 42ks of integration with the Subaru/OHS spectrograph, no emission-linefeatures are detected in the J and H bands, which confirms that SDFJ132440.6+273607 is neither an active galactic nucleus nor a low-zemission-line object. We obtained a 2 σ upper limit of9.06×10-18 ergs s-1 cm-2 on theHe II λ1640 emission-line flux, which corresponds to a luminosityof 4.11×1042 ergs s-1. This upper limit onthe He II λ1640 luminosity implies that the upper limit on thePopulation III star formation rate is in the range 4.9-41.2Msolar yr-1 if Population III stars suffer no massloss and in the range 1.8-13.2 Msolar yr-1 ifstrong mass loss is present. The nondetection of He II in SDFJ132440.6+273607 at z=6.33 may thus disfavor weak feedback models forPopulation III stars.Based on data collected at the Subaru Telescope, which is operated bythe National Astronomical Observatory of Japan.
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