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On the Origin of the Ly $\alpha$ Damping Wing in Galaxies at $8\le z \le 10$ : Explorations using the NINJA Simulations

By combining NINJA cosmological simulations with idealized ionized-bubble models, this study demonstrates that while the intergalactic and circumgalactic media contribute to Ly $\alpha$ damping wings in galaxies at $z=8$ –10, the strongest observed absorbers require additional contributions from partially ionized gas within the virial radius and unresolved stellar birth clouds, thereby establishing these features as unique probes of the ionization states across the ISM, CGM, and IGM.

Original authors: Sukanya Mallik, Raghunathan Srianand, Nishikanta Khandai

Published 2026-08-07

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CC BY 4.0

Original authors: Sukanya Mallik, Raghunathan Srianand, Nishikanta Khandai

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, dark ocean that existed right after the Big Bang. For hundreds of millions of years, this ocean was filled with a thick, invisible fog made of neutral hydrogen gas. It was so thick that light couldn't travel through it easily; the universe was effectively "foggy" and opaque. Then, the first stars and galaxies ignited like lighthouses in the dark. Their intense ultraviolet light began to blast through the fog, burning it away and turning the neutral gas into a clear, ionized state. This massive cleanup operation is called the "Epoch of Reionization."

Astronomers are obsessed with figuring out exactly when and how this happened. They look at ancient galaxies and try to measure how much of that original fog is still clinging to them. One of their best tools is a specific type of light signature called the "Lyman-alpha damping wing." Think of this like a shadow cast by the fog. If the fog is thick, the shadow is long and dark; if the fog is thin, the shadow is faint. By measuring these shadows, scientists hope to map the history of the universe's clearing. However, there's a catch: the shadow isn't just cast by the distant fog. The galaxy itself, and the gas swirling right around it, can cast its own shadow too. It's like trying to measure the fog in a valley while standing in a dense forest; the trees right next to you make it hard to tell how thick the fog is further away.

This is where a new study comes in, using powerful computer simulations to untangle this cosmic knot. The researchers, using a suite of simulations called "NINJA," wanted to figure out exactly how much of the "foggy shadow" seen in distant galaxies comes from the vast universe versus how much comes from the galaxy's own neighborhood. They built three different virtual scenarios to test their ideas. First, they imagined a galaxy sitting in a completely uniform fog. Second, they imagined the galaxy sitting inside a giant, clear bubble of ionized gas (like a bubble of air in water) surrounded by fog. Third, they added a twist: what if there was still some thick, neutral gas hiding inside that clear bubble, right around the galaxy's core?

The results were revealing. The simulations showed that galaxies are indeed surrounded by a dense cloud of gas extending about 60 to 100 kiloparsecs (a distance roughly 200,000 to 300,000 light-years) out. The size of this cloud depends on how massive the galaxy's "home" (its dark matter halo) is, but it doesn't change much between 8 and 10 billion years after the Big Bang. When the team looked at the "shadows" (the damping wings), they found that if the universe was mostly foggy, the shadows would be huge. But the observations from the James Webb Space Telescope (JWST) show a mix: some galaxies have faint shadows, while others have incredibly deep, dark shadows.

The simulations suggest that the faint shadows can be explained if galaxies are sitting in clear bubbles about 50 to 400 kiloparsecs wide. However, the deepest shadows—the ones that indicate a massive amount of gas—cannot be explained just by the fog in the universe or even by the gas in the clear bubble. The paper argues that to get those super-dark shadows, you need a significant amount of neutral gas hiding inside the galaxy's own immediate vicinity, likely within the galaxy's core or in the clouds where stars are being born. In fact, the simulations suggest that even with all the gas they modeled, they still couldn't quite reproduce the very strongest shadows seen in the real data. This hints that there might be even more hidden gas in "stellar birth clouds" that the simulations didn't fully capture.

Ultimately, the study concludes that we can't just look at the distant fog to understand the universe's history. To truly decode the shadows cast by these ancient galaxies, we have to understand the complex, messy, and dense gas right around the galaxies themselves. The strongest shadows are a unique fingerprint of the ionization state of the gas inside and immediately around the galaxy, proving that the story of reionization is written not just in the vast spaces between stars, but in the turbulent neighborhoods where stars are born.

Technical Summary: On the Origin of the Lyα Damping Wing in Galaxies at 8 ≤ z ≤ 10

Problem Statement
The Epoch of Reionization (EoR) is characterized by the transition of the intergalactic medium (IGM) from neutral to ionized. While the Lyα damping wing observed in high-redshift ( $7 \le z \le 14$ ) galaxy spectra via the James Webb Space Telescope (JWST) offers a powerful probe of this era, interpreting these features is complex. The damping wing shape is degenerate, arising from a combination of absorption by the diffuse IGM, the circumgalactic medium (CGM), and the interstellar medium (ISM) of the source galaxy itself. Current observations reveal a high incidence of damped Lyα (DLA) systems with high neutral hydrogen column densities ( $N_{\text{H I}}$ ), yet it remains unclear to what extent these features are driven by the global IGM neutral fraction ( $x_{\text{HI}}$ ) versus local gas reservoirs around galaxies. Existing models often struggle to simultaneously reproduce the observed incidence of strong absorbers and the median column densities without physically motivated constraints on the ionization state of gas at multiple scales.

Methodology
The authors utilize the Ninja (NISER) suite of large-scale cosmological hydrodynamical simulations, which incorporate star formation, stellar feedback, and black hole growth but do not include self-consistent radiative transfer (RT). To model the EoR environment, the authors employ an idealized post-processing approach combining these simulations with a toy-model of ionized bubbles.

The study generates mock Lyα absorption spectra at redshifts $z=8$ and $z=10$ using three distinct scenarios to isolate contributions from different gas components:

Case I: Absorption arises solely from a uniformly ionized (or partially ionized) diffuse IGM with a global neutral fraction $x_{\text{HI}}$ .
Case II: Galaxies are embedded in spherical, fully ionized H II regions of radius $R_{\text{H II}}$ (ranging from 0 to 400 pkpc). Absorption includes contributions from the IGM and the over-dense gas surrounding the halo (CGM) outside the bubble, but assumes no neutral gas within the bubble.
Case III: A modification of Case II where residual neutral gas with a specific neutral fraction ( $x^{\text{V}}_{\text{HI}}$ ) is included within the virial radius ( $R_{\text{vir}}$ ) of the halo, while the gas between $R_{\text{vir}}$ and $R_{\text{H II}}$ remains fully ionized.

The simulations use three boxes of varying resolution and volume ( $L50N640$ , $L50N1008$ , and $L140N1008$ ) to assess numerical convergence and halo mass dependence. Mock spectra are convolved with a Gaussian profile to match the low spectral resolution ( $R \sim 100-300$ ) of JWST/NIRSpec PRISM mode and include Poissonian noise (SNR=30). The strength of absorption is quantified by the parameter $x_p$ (velocity separation at 10% transmission), which is mapped to an equivalent neutral hydrogen column density ( $N_{\text{H I}}$ ).

Key Contributions and Results
The paper presents a systematic quantification of how IGM, CGM, and ISM contributions shape the Lyα damping wing:

Gas Over-density and Halo Mass: The simulations reveal that galaxies are surrounded by over-dense gas extending to $\sim 60-100$ pkpc. The extent of this over-dense region ( $R_{\text{eq}}$ ) scales with halo mass (increasing from $\sim 66$ pkpc for lower-mass halos to $\sim 105$ pkpc for more massive halos) but shows little evolution between $z=8$ and $z=10$ .
IGM Contribution (Case I): Models with a uniformly ionized IGM or low neutral fractions ( $x_{\text{HI}} = 0.1$ ) fail to reproduce the observed incidence of strong DLA systems. Even with a fully neutral IGM ( $x_{\text{HI}} = 1.0$ ), the simulated IGM sightlines rarely exceed $N_{\text{H I}} > 10^{22} \text{ cm}^{-2}$ . This suggests that the IGM alone cannot explain the strongest absorbers observed in JWST data.
Role of H II Regions (Case II): The presence of ionized bubbles significantly alters absorption statistics.
- If $R_{\text{H II}} \approx 0$ (no bubbles), the models predict $>90\%$ of galaxies have $N_{\text{H I}} > 10^{21} \text{ cm}^{-2}$ , which exceeds observational constraints, implying that finite-sized ionized regions must exist around a significant fraction of galaxies.
- Conversely, if $R_{\text{H II}} > 50$ pkpc, the models fail to reproduce the strongest absorbers ( $N_{\text{H I}} > 10^{22} \text{ cm}^{-2}$ ), even with a neutral IGM. This indicates that large ionized bubbles suppress the damping wing too much to match observations of strong DLAs.
Residual Neutral Gas (Case III): Including residual neutral gas within the virial radius ( $x^{\text{V}}_{\text{HI}} > 0$ ) is necessary to produce the strongest damped Lyα absorbers. However, even with fully neutral gas inside the virial radius, the predicted incidence of $N_{\text{H I}} > 10^{22} \text{ cm}^{-2}$ systems remains below observational estimates for the halo masses resolved in the simulations.
Mass Dependence: The inferred $N_{\text{H I}}$ correlates positively with both stellar and halo mass. Stronger absorption is preferentially found in more massive halos due to larger gas reservoirs and denser environments.

Significance and Conclusions
The paper concludes that the observed Lyα damping wings in galaxies at $z \gtrsim 8$ are composite features resulting from absorption in the IGM, the CGM, and the ISM. The authors assert that:

IGM Limitations: The neutral fraction of the diffuse IGM alone cannot explain the high incidence of strong damped Lyα absorbers observed by JWST.
Necessity of Local Gas: Reproducing the strongest absorbers requires contributions from neutral gas within the galaxy's virial radius and its immediate over-dense surroundings.
Missing Physics: The discrepancy between the simulated incidence of strong absorbers (even with virial radius neutral gas) and observations suggests that the observed galaxies may reside in more massive halos than those resolved in the current simulations, or that additional unresolved neutral gas (e.g., in stellar birth clouds) contributes significantly to the absorption.
Methodological Requirement: Robust constraints on the IGM neutral fraction ( $x_{\text{HI}}$ ) require unified models that simultaneously account for the ionization state of the ISM, CGM, and IGM. The study highlights the need to connect small-scale radiative transfer calculations with large-scale reionization simulations to fully interpret low-resolution JWST spectra.

The authors emphasize that their results are numerically robust across different simulation resolutions and volumes, providing a baseline for future work that integrates multi-scale radiative transfer to resolve the degeneracies in EoR observations.

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