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 z > 2 (z > 5.3), finding 52 (26) galaxies with Balmer line ratios more than 1σ 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 photoionisation 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 Hα/Hβ versus Hγ/Hβ trend of density-bounded models is robust to changes in stellar age of the ionising source, gas density, and ionisation parameter; however, increasing the stellar metallicity drives a turnover in the trend. This is due to stronger stellar absorption features around Lyγ reducing Hβ fluorescence, allowing density-bounded models to account for all observed Balmer line ratios. ABEs show higher [O iii]/[O ii], have steeper ultra-violet slopes, are fainter, and are more preferentially Lyα 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.