Probing Population III Stars in Galaxy IOK-1 at z = 6.96 through He II
  Emission
  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 < 6% of the total star
formation. This measurement places the strongest limit yet on metal-free star
formation at high redshift, although the exact conversion from He II luminosity
to Pop III SFR is highly uncertain due to the unknown IMF, stellar evolution,
and photoionization effects. Although we have not detected He II \lambda 1640
at more than the 1.2\sigma level, our work suggests that a > 3\sigma level
detection is possible with JWST.
