Neutral hydrogen in and around galaxies during the Epoch of Reionization

TL;DR

This work uses high-resolution serra zoom-in simulations to quantify neutral hydrogen along sightlines to star-forming regions in ~100 galaxies during the Epoch of Reionization. It demonstrates that the HI column-density distribution is broad and dominated by dense ISM gas within ~1 kpc of star-forming regions, with a weaker redshift evolution at fixed halo mass and a median around $N_HI\sim10^{21}$–$10^{22}$ cm$^{-2}$. The findings imply that JWST-detected strong DLAs at $z>5$ largely arise from the inner ISM of massive halos and that the local HI distribution can serve as informative priors for interpreting IGM damping wings at higher redshift. The work also predicts observable metal absorption signatures and correlates extreme DLAs with halos hosting multiple nearby galaxies, offering concrete tests for upcoming spectroscopy and a framework to disentangle local and IGM contributions in Lyα damping wings.

Abstract

JWST spectra revealing Lyman-$α$ (Ly$α$) absorption in $z\sim 5-14$ galaxies offer a unique probe of reionization's earliest stages. However, disentangling absorption by the increasingly neutral intergalactic medium (IGM) from that in galaxies' interstellar and circumgalactic medium (ISM, CGM) remains challenging due to the poorly constrained nature of neutral hydrogen (HI) in and around galaxies at these redshifts. We use the SERRA high-resolution cosmological zoom-in simulations to characterize the HI distribution and its evolution along sightlines to star-forming regions during reionization, to interpret the contribution of local HI (ISM+CGM) to Ly$α$ absorption in $z>5$ spectra. We analyze $\sim 100$ $z=6-9.5$ galaxies, generating mock sightlines from each galaxy's star formation peak. We study the sightline distribution of HI column densities ($N_{\rm HI}$) and its variation with radius, halo mass and redshift. We find broad sightline variation in $N_{\rm HI}$ (0.5-1.5dex) due to complex ISM morphology driven by bursty star formation, with median $\log(N_{\rm HI}/{\rm cm}^{-2}) \simeq 21-22$. Dense ISM gas is the dominant origin of damped Ly$α$ absorption (DLA) systems along sightlines towards star-forming regions, outweighing gas in the CGM, filaments and proximate absorbers. Median $N_{\rm HI}$ increases with halo mass, scaling roughly with the virial radius, as expected due to larger potentials and more extended CGM, but shows negligible redshift evolution at fixed halo mass. This suggests post-reionization $N_{\rm HI}$ distributions may provide useful priors to interpret IGM damping wings at higher redshifts. To interpret strong $z>5$ DLA candidates found by JWST, we investigate $N_{\rm HI}>10^{22}{\rm cm}^{-2}$ sightlines. These trace dense, metal-enriched ISM within <1 kpc of massive halos, a scenario testable with higher-resolution spectroscopy.

Figures (7)

• Figure 1: Maps of Amana, typical star forming $z=7$ galaxy of $M_\star \sim 10^{9.5}{\rm M}_{\odot}$ and ${\rm SFR}\sim 15~{\rm M}_{\odot}\,{\rm yr}^{-1}$ from the serra simulation. The three panels show quantities along the $z$ direction: the neutral hydrogen column density ($N_{HI}$), the stellar mass surface density ($\Sigma_\star$), and the sightline mass-averaged gas metallicity ($Z$). Each map covers a 100 kpc field of view, with an inset showing the ISM by zooming on the inner 5 kpc.
• Figure 2: Top: HI density along three different sightlines from the centre of the galaxy Amana. The resulting column density along each sightline targeting the same galaxy can thus vary significantly. Bottom: Probability distributions of HI column densities along 3000 random sightlines towards the central galaxy (blue) and the same distribution only considering the contribution of the CGM to the column density (lilac) obtained by excluding the inner 2.5 kpc. The black histogram show the results of GRB DLA at $z\sim 2-6$ by Tanvir19.
• Figure 3: Star formation history of a typical massive galaxy in serra (centre). For three specific timesteps during its evolution, during and following a strong burst of star formation, we show the ISM column density maps over 5 kpc size boxes (top) and column density distributions (bottom). The red crosses in the maps pinpoint the location of the peak of star formation.
• Figure 4: Neutral hydrogen column density metrics for each galaxy in the sample, as a function of key physical properties. Each row corresponds to a different quantity derived from the $N_\mathrm{HI}$N_HI$\xspace$ distributions along random sightlines: median, standard deviation of $\log(N_\mathrm{HI}$N_HI$\xspace/\mathrm{cm}^{-2})$, and fraction of sightlines with $N_\mathrm{HI}$N_HI$\xspace > 10^{20},\mathrm{cm^{-2}}$ and $N_\mathrm{HI}$N_HI$\xspace > 10^{22},\mathrm{cm^{-2}}$. Each column shows results versus (from left to right): halo mass, star formation rate (averaged over the last 3 Myr), burstiness ratio ($\rm SFR_3/SFR_{50}$), stellar mass, and ISM metallicity. Points are color-coded by halo mass. A clear trend with halo mass is visible for all $N_\mathrm{HI}$ metrics, while burstiness and metallicity show no significant correlation with $N_\mathrm{HI}$N_HI$\xspace$ properties. The dashed line in the first panel shows the result of the power-law fit of $N_\mathrm{HI}$N_HI$\xspace\propto M_h^{0.38}$. The large intrinsic scatter present in all the relations highlights the complex and inhomogeneous ISM structure within individual galaxies.
• Figure 5: Fraction of sightlines with column density above thresholds $\rm 10^{20} cm^{-2}, 10^{21} cm^{-2}, 10^{22} cm^{-2}$ for all galaxies in our sample from serra, color coded with their halo mass. The figures illustrate a clear radial decline, with the inner ISM contributing most significantly to the probability of encountering high column densities. A strong dependence on halo mass is also evident across all thresholds.