GNHeII J1236+6215: A He II λ1640 Emitting and Potentially LyC Leaking Galaxy at z = 2.9803 Unveiled through JWST and Keck Observations

He II λ1640 emission in galaxies indicates the presence of sources that produce extreme ionizing photons. Here we report the discovery of a He II λ1640 emitting galaxy, GNHeII J1236+6215, at z = 2.9803 in the GOODS-north field. We use photometry in 17 wave bands from near-UV to infrared to characterize the galaxy spectral energy distribution (SED) and combine Keck Low-resolution Imaging Spectrometer and JWST NIRSpec spectra to identify 15 emission lines including He II λ1640. We infer that the He+ ionization in the galaxy could be driven by small pockets of young Population III stars or low-metallicity very massive stars rather than active galactic nuclei or metal-rich Wolf–Rayet stars. The galaxy has a highly ionized interstellar medium (ISM) ([O III]5007/[O II]3727 = 7.28 ± 0.11, [S III]/[S II] = 1.97 ± 0.48, and detected Lyα, Hα, Hβ, Hγ lines), little reddening by dust (E(B − V) = 0.04 ± 0.12), low metallicity (12 + log(O/H) = 7.85 ± 0.22), and high star formation rate (SFRSED = 12.2 ± 2.0 M⊙ yr−1). In addition to these ISM conditions, we also notice a significant [S II] deficiency ([S II]6718,6732/Hα = 0.08 ± 0.02, Δ[S II] = −0.12), which may indicate the presence of density-bounded optically thin H II regions that, combined with the low-dust extinction, favor leaking of ionizing Lyman continuum photons. Our best-fit SED model also infers a high nebular ionization (log U = −2.0) and a low stellar mass M = 7.8 ± 3.1 × 108M⊙. This discovery not only adds one important object to the known sample of high-redshift He II emitters but also highlights a potential connection between He+ ionization and favorable ISM conditions for the leakage of ionizing photons from galaxies.