Probing Population III Stars in Galaxy IOK-1 at z = 6.96 through He II Emission

TL;DR

This study uses deep narrowband imaging to set the most stringent limits yet on Population III star formation in a high-redshift galaxy, finding that such stars contribute less than 6% of total star formation.

Contribution

It provides the deepest He II 40 emission limits at high redshift and discusses the potential for future JWST detections of Pop III stars.

Findings

He II flux upper limit of 1.2 ^{-18} erg/s/cm^2

Pop III stars contribute less than 6% of total star formation

Potential for >3 detection with JWST

Abstract

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.