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?

Abstract

We present He i /H β 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 i λ 4471 and He ii λ 4686 to He i λ 7065, and comparing with 68 local dwarf galaxies. We find that these high- z galaxies present strong He i emission with He i /H β flux ratios generally larger than those of local dwarf galaxies. We derive He/H with all of the detected He i , 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 of n _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.

Keywords

• Galaxy chemical evolution
• Galaxy evolution
• High-redshift galaxies
• Chemical abundances
• Chemical enrichment
• Galaxy formation