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

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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.