2025MNRAS.540..190M

The peculiar nebular emission displayed by galaxies in the early Universe presents a unique opportunity to gain insight into the regulation of star formation in extreme environments. We investigate 500 (109) galaxies with deep NIRSpec/PRISM observations from the JADES survey at $z>2$ ($z>5.3$), finding 52 (26) galaxies with Balmer line ratios more than $1\sigma$ inconsistent with Case B recombination. These anomalous Balmer emitters (ABEs) cannot be explained by dust attenuation, indicating a departure from Case B recombination. To address this discrepancy, we model density-bounded nebulae with the photoionization code CLOUDY. Density-bounded nebulae show anomalous Balmer line ratios due to Lyman line pumping and a transition from the nebulae being optically thin to optically thick for Lyman lines with increasing cloud depth. The H $\alpha$/H $\beta$ versus H $\gamma$/H $\beta$ trend of density-bounded models is robust to changes in stellar age of the ionizing source, gas density, and ionization parameter; however, increasing the stellar metallicity drives a turnover in the trend. This is due to stronger stellar absorption features around Ly $\gamma$ reducing H $\beta$ fluorescence, allowing density-bounded models to account for all observed Balmer line ratios. ABEs show higher [O III]/[O II], have steeper ultraviolet slopes, are fainter, and are more preferentially Ly $\alpha$ emitters than galaxies which are consistent with Case B and little dust. These findings suggest that ABEs are galaxies that have become density-bounded during extreme quenching events, representing a transient phase of $\sim$20 Myr during a fast breathing mode of star formation.