The density-bounded twilight of starbursts in the early Universe

TL;DR

This study investigates early Universe starburst galaxies, revealing that some are density-bounded during quenching events, which affects their emission lines and star formation regulation, using deep NIRSpec observations and photoionisation modeling.

Contribution

It demonstrates that anomalous Balmer line ratios in early galaxies can be explained by density-bounded nebulae, highlighting a transient phase of star formation quenching.

Findings

52 galaxies show anomalous Balmer ratios inconsistent with Case B.

Density-bounded models explain the observed line ratios across various parameters.

ABEs are more likely to be Lyα emitters and have properties indicating density bounding during quenching.

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 $1\sigma$ 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 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 H$\alpha$/H$\beta$ versus H$\gamma$/H$\beta$ 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 Ly$\gamma$ reducing H$\beta$ fluorescence, allowing density-bounded models to account for all observed Balmer line ratios. ABEs show higher [OIII]/[OII], have steeper ultra-violet 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.