Extreme damped Lyman-$α$ absorption in young star-forming galaxies at $z=9-11$ (2306.00647v1)

Abstract: The onset of galaxy formation is thought to be initiated by the infall of neutral, pristine gas onto the first protogalactic halos. However, direct constraints on the abundance of neutral atomic hydrogen (HI) in galaxies have been difficult to obtain at early cosmic times. Here we present spectroscopic observations with JWST of three galaxies at redshifts $z=8.8 - 11.4$, about $400-600$ Myr after the Big Bang, that show strong damped Lyman-$\alpha$ absorption ($N_{\rm HI} > 10^{22}$ cm$^{-2}$) from HI in their local surroundings, an order of magnitude in excess of the Lyman-$\alpha$ absorption caused by the neutral intergalactic medium at these redshifts. Consequently, these early galaxies cannot be contributing significantly to reionization, at least at their current evolutionary stages. Simulations of galaxy formation show that such massive gas reservoirs surrounding young galaxies so early in the history of the universe is a signature of galaxy formation in progress.

• The paper identifies extreme damped Lyman-α absorption in z=9–11 galaxies with hydrogen column densities exceeding 10^22 cm⁻².
• The paper demonstrates that these early star-forming galaxies harbor massive neutral hydrogen reservoirs, implying limited ionizing photon escape.
• The paper supports theoretical models of early galaxy formation and calls for expanded high-resolution observations to refine reionization timelines.

Overview of Extreme Damped Lyman-alpha Absorption in Early Galaxies

This paper explores the presence of extreme damped Lyman-alpha (DLA) absorption in three early star-forming galaxies at redshifts $z \approx 9-11$, utilizing observations from the James Webb Space Telescope (JWST). The paper provides crucial insights into galaxy formation shortly after the Big Bang, approximately 400-600 Myr later. Through spectroscopic observations, the researchers detect significant neutral atomic hydrogen reservoirs ($N_{\text{H\,i}} > 10^{22} \text{cm}^{-2}$), much higher than anticipated from the neutral intergalactic medium (IGM) at these epochs.

Key Findings

• High Column Densities: The research reveals extreme DLAs with hydrogen column densities an order of magnitude greater than previously observed in similar contexts, specifically exceeding $10^{22} \text{cm}^{-2}$. This suggests a substantial build-up of pristine, neutral gas in these galaxies.
• Absence of Significant Ionizing Contribution: The presence of massive shells of neutral gas implies these galaxies are not significant contributors to the reionization process at their current evolutionary stage.
• Comparison with Other Epochs: When compared to similar emission-line galaxies at $z \approx 0$ and previous DLA detection in Lyman-break galaxies at $z \approx 3$, these early galaxies display much denser and massive hydrogen reservoirs.
• Theoretical Implications: The findings assert support for theoretical models predicting substantial neutral gas during initial star formation phases, complicating the escape of ionizing radiation critical for reionization.

Implications and Future Directions

The presence of extensive and dense neutral reservoirs raises questions about galaxy formation and the timeline of universal reionization. If such gas-rich environments are common among galaxies around $z = 9-11$, they significantly impact how models predict the contributions of high-redshift galaxies to reionization. The research underscores the need for in-depth surveys of early galaxies to refine our understanding of their role in cosmic reionization.

Future directions might include:

1. Expanded Observational Campaigns: Investigations involving larger samples to assess the commonality of such neutral gas reservoirs and their roles in reionization.
2. High-Resolution Simulations: Improved simulations incorporating these findings to model the evolution of neutral and ionized regions and refine reionization timelines.
3. Daya on Ionizing Photon Escape: Exploring different scenarios and conditions under which these galaxies might become significant contributors to reionization.

Overall, this paper presents pivotal observational evidence about the state of early galaxies and offers a new perspective on the mechanisms of galaxy formation and the reionization epoch. The findings suggest that while these galaxies contain massive amounts of neutral hydrogen, they may not be primary agents in ionizing the early universe, necessitating a revision of current models concerning early cosmic evolution and reionization.