Probing Population III Stars in Galaxy IOK-1 at z = 6.96 through He II Emission
(13 votes over all institutions)
The He II \lambda 1640 emission line has been suggested as a direct probe of Population III (Pop III) stars at high-redshift, since it can arise from highly energetic ionizing photons associated with hot, metal free stars. We use the HST WFC3/F130N IR narrowband filter to probe He II \lambda 1640 emission in galaxy IOK-1 at z=6.96. The sensitivity of this measurement is >5x deeper than for previous measurements. From this deep narrowband imaging, combined with broadband observations in the F125W and F160W filters, we find the He II flux to be 1.2+/- 1.0x 10^-18 ergs/s/cm^2, corresponding to a 1\sigma upper limit on the Pop III star formation rate (SFR) of ~ 0.5 M_sun/yr for the case of a Salpeter IMF with 50-500M_sun and mass loss. Given that the broadband measurements can be fit with a UV continuum spectral flux density of ~ 4.85x 10^-10x \lambda ^-2.46 ergs/s/cm^2/A, which corresponds to an overall SFR of ~16+/-2.6 M_sun/yr, massive Pop III stars represent 3\sigma level detection is possible with JWST.
