Constraining very high mass population III stars through He II emission in galaxy BDF-521 at z = 7.01

Numerous theoretical models have long proposed that a strong He II λ1640 emission line is the most prominent and unique feature of massive Population III (Pop III) stars in high-redshift galaxies. The He II λ1640 line strength can constrain the mass and initial mass function (IMF) of Pop III stars. We use F132N narrowband filter on the Hubble Space Telescope's (HST) Wide Field Camera 3 to look for strong He II λ1640 emission in the galaxy BDF-521 at z = 7.01, one of the most distant spectroscopically confirmed galaxies to date. Using deep F132N narrowband imaging, together with our broadband imaging with F125W and F160W filters, we do not detect He II emission from this galaxy, but place a 2σ upper limit on the flux of 5.3 × 10 ^-19. This measurement corresponds to a 2σ upper limit on the Pop III star formation rate (SFR_PopIII) of ∼0.2 M_⊙ yr^-1, assuming a Salpeter IMF with 50 ≲ M/M_⊙ ≲ 1000. From the high signal-to-noise broadband measurements in F125W and F160W, we fit the UV continuum for BDF-521. The spectral flux density is ∼ 3.6 × 10^-11 × λ^-2.32 erg s^-1 cm^-2 Å^-1, which corresponds to an overall unobscured SFR of ∼5 M_⊙ yr^-1. Our upper limit on SFR_PopIII suggests that massive Pop III stars represent ≲4% of the total star formation. Further, the HST high-resolution imaging suggests that BDF-521 is an extremely compact galaxy, with a half-light radius of 0.6 kpc.