The density-bounded twilight of starbursts in the early Universe

ABSTRACT 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 z2 (z5. 3), finding 52 (26) galaxies with Balmer line ratios more than 1 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 /H versus H /H 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 reducing H 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 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 20 Myr during a fast breathing mode of star formation.