Probing population III stars in galaxy IOK-1 at z = 6.96 through He II emission

The He II λ1640 emission line has been suggested as a direct probe of Population III (PopIII) stars at high redshift, since it can arise from highly energetic ionizing photons associated with hot, metal-free stars. We use the Hubble Space Telescope WFC3/F130N IR narrowband filter to probe He II λ1640 emission in galaxy IOK-1 at z = 6.96. The sensitivity of this measurement is ≳5× 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 ± 1.0) × 10 ^-18 erg s^-1 cm^-2, corresponding to a 1σ upper limit on the PopIII star formation rate (SFR) of ∼0.5 M _⊙ yr^-1 for the case of a Salpeter initial mass function (IMF) with 50 ≲ M/M_⊙ ≲ 1000 and mass loss. Given that the broadband measurements can be fit with a UV-continuum spectral flux density of ∼4.85 × 10^-10 × λ ^-2.46 erg Ås^-1 cm^{-2 -1}, which corresponds to an overall SFR of ∼16^+2.6 _{- 2.6} M _⊙ yr^-1, massive PopIII 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 PopIII SFR is highly uncertain due to the unknown IMF, stellar evolution, and photoionization effects. Although we have not detected He II λ1640 at more than the 1.2σ level, our work suggests that a ≳ 3σ level detection is possible with the James Webb Space Telescope.