A Mysteriously Tight H$\alpha$-[O III] Correlation and Non-Case B Balmer Decrements Revealed by the Spectra from the James Webb Space Telescope NIRSpec Instrument

Abstract

We report an extremely tight, linear relation between ${\rm H\alpha}$ and [O~III] line fluxes in logarithm, discovered using a large sample of low and mid-resolution spectra (totaling 563) obtained by the James Webb Space Telescope (JWST) NIRSpec instrument in three widely separated extragalactic fields. While a certain correlation between ${\rm H\alpha}$ and [O~III] is expected for star forming galaxies, such a log-linear and tight (dispersion of $\sim$0.1 dex) trend is hard to explain because dust reddening would skew any intrinsic relation between the two. Furthermore, another surprising finding emerges from investigating the dust reddening properties of these galaxies. We find that the classic method of using the Balmer decrements under the standard Case B assumption does not work: a high fraction (${\sim30\%}$) of our objects have ${\rm H\alpha}$/${\rm H\beta}$ line ratios even smaller than the canonical Case B ratio of 2.86. Such a high fraction of non-Case B Balmer decrements is also present in other JWST and ground-based spectroscopic studies, but the universal applicability of the Case B assumption was not questioned until recently. The mysterious ${\rm H\alpha}$--[O~III] correlation and the high fraction of non-Case B Balmer decrements, which may or may not be related, should be further investigated to put our spectral analysis onto a more solid footing.