Strong He I Emission Lines in High N/O Galaxies at z ∼ 6 Identified in JWST Spectra: High He/H Abundance Ratios or High Electron Densities?

We present HeI/Hβ-flux and He/H-abundance ratios in three JWST galaxies with significant constraints on N/O-abundance ratios, GS-NDG-9422, RXCJ2248-ID, and GLASS150008 at z∼ 6 mostly with the spectroscopic coverage from HeIλ4471 and HeIIλ4686 to HeIλ7065, comparing with 68 local-dwarf galaxies. We find that these high-z galaxies present strong HeI emission with HeI/Hβ flux ratios generally larger than those of local-dwarf galaxies. We derive He/H with all of the detected HeI, HeII, and 2-3 hydrogen Balmer lines in the same manner as the local He/H determination conducted for cosmology studies. These high-z galaxies show He overabundance He/H≳ 0.10 or high electron density n_e∼ 10^3-4 cm^-3 much larger than local values at low O/H, 12+log(O/H)=7-8. In contrast, we obtain low He/H and n_e values for our local-dwarf galaxies by the same technique with the same helium and hydrogen lines, and confirm that the difference between the high-z and local-dwarf galaxies are not mimicked by systematics. While two scenarios of 1) He overabundance and 2) high electron density are not clearly concluded, we find that there is a positive correlation on the He/H-N/O or n_e-N/O plane by the comparison of the high-z and local-dwarf galaxies. The scenario 1) suggests that the overabundant helium and nitrogen are not explained by the standard chemical enrichment of core-collapse supernovae, but the CNO-cycle products and equilibrium ratios, respectively. The scenario 2) indicates that the strong helium lines are originated from the central dense clouds of the high-z galaxies by excessive collisional excitation.