Strong He I Emission Lines in High N/O Galaxies at Identified in JWST Spectra: High He/H Abundance Ratios or High Electron Densities?
arXiv:2405.01823
Abstract
We present HeI/H-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 mostly with the spectroscopic coverage from HeI4471 and HeII4686 to HeI7065, comparing with 68 local-dwarf galaxies. We find that these high- galaxies present strong HeI emission with HeI/H flux ratios generally larger than those of local-dwarf galaxies. We derive He/H with all of the detected HeI, HeII, and hydrogen Balmer lines in the same manner as the local He/H determination conducted for cosmology studies. These high- galaxies show He overabundance He/H or high electron density cm much larger than local values at low O/H, . In contrast, we obtain low He/H and 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- 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/O plane by the comparison of the high- 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- galaxies by excessive collisional excitation.
19 pages, 12 figures, accepted for publication in ApJ