2024ApJ...974..266Y

We present He I/Hβ flux and He/H abundance ratios in three James Webb Space Telescope 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 He Iλ4471 and He IIλ4686 to He Iλ7065, and comparing with 68 local dwarf galaxies. We find that these high-z galaxies present strong He I emission with He I/Hβ flux ratios generally larger than those of local dwarf galaxies. We derive He/H with all of the detected HeI, He II, 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 ofn_e ∼ 10^3–4 cm^–3 much larger than local values at low O/H, $12+\mathrm{log}({\rm{O}}/{\rm{H}})=7-8$. 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 is 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 between the He/H-N/O or n_e-N/O plane by the comparison of the high-z and local dwarf galaxies. Scenario (1) suggests that the overabundant helium and nitrogen are not explained by the standard chemical enrichment of core-collapse supernovae, but by the CNO-cycle products and equilibrium ratios, respectively. Scenario (2) indicates that the strong helium lines originated from the central dense clouds of the high-z galaxies by excessive collisional excitation.