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?

Yanagisawa, Hiroto,   Ouchi, Masami,   Watanabe, Kuria,   Matsumoto, Akinori,   Nakajima, Kimihiko,   Yajima, Hidenobu,   Nagamine, Kentaro,   Takahashi, Koh,   Nakane, Minami,   Tominaga, Nozomu,   Umeda, Hiroya,   Fukushima, Hajime,   Harikane, Yuichi,   Isobe, Yuki,   Ono, Yoshiaki,   Xu, Yi,   & Zhang, Yechi

要旨

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 I λ4471 and He II λ4686 to He I λ7065, 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 of 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 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.