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

TL;DR

This study analyzes JWST spectra of high-redshift galaxies, revealing strong helium emission lines that suggest either high helium abundance or elevated electron densities, differing from local dwarf galaxies and impacting understanding of early galaxy evolution.

Contribution

It provides the first detailed comparison of HeI emission and He/H ratios in $z\,\sim6$ galaxies with local dwarfs, proposing two possible explanations for the observed strong HeI lines.

Findings

High-$z$ galaxies show HeI/H$eta$ ratios larger than local dwarfs.

High-$z$ galaxies exhibit He/H > 0.10 or high electron densities (~10^{3-4} cm$^{-3}$).

A positive correlation exists between He/H or electron density and N/O ratios.

Abstract

We present HeI/H$\beta$-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 $z\sim 6$ mostly with the spectroscopic coverage from HeI$\lambda$4471 and HeII$\lambda$4686 to HeI$\lambda$7065, comparing with 68 local-dwarf galaxies. We find that these high-$z$ galaxies present strong HeI emission with HeI/H$\beta$ 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$\gtrsim 0.10$ or high electron density $n_\mathrm{e}\sim 10^{3-4}$ cm$^{-3}$ much larger than local values at low O/H, $12+\log \mathrm{(O/H)}=7-8$. In contrast, we obtain low He/H and $n_\mathrm{e}$ 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 $n_\mathrm{e}$-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.