The density-bounded twilight of starbursts in the early Universe
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2024
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| author | McClymont, William Tacchella, Sandro D'Eugenio, Francesco Witten, Callum Ji, Xihan Smith, Aaron Maiolino, Roberto Arribas, Santiago Scholtz, Jan Simmonds, Charlotte Witstok, Joris |
| author_facet | McClymont, William Tacchella, Sandro D'Eugenio, Francesco Witten, Callum Ji, Xihan Smith, Aaron Maiolino, Roberto Arribas, Santiago Scholtz, Jan Simmonds, Charlotte Witstok, Joris |
| contents | 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 H$α$/H$β$ versus H$γ$/H$β$ 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$γ$ reducing H$β$ 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$α$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2405_15859 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | The density-bounded twilight of starbursts in the early Universe McClymont, William Tacchella, Sandro D'Eugenio, Francesco Witten, Callum Ji, Xihan Smith, Aaron Maiolino, Roberto Arribas, Santiago Scholtz, Jan Simmonds, Charlotte Witstok, Joris Astrophysics of Galaxies 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 H$α$/H$β$ versus H$γ$/H$β$ 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$γ$ reducing H$β$ 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$α$ 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. |
| title | The density-bounded twilight of starbursts in the early Universe |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2405.15859 |