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

Ly alpha damping wing measurements of galaxies at 7 at most z at most14 with the JWST provide a powerful probe of the Epoch of Reionization. We combine the large-scale cosmological hydrodynamical NINJA simulations with an idealized ionized-bubble model at z=8 and 10 to quantify the contributions of the intergalactic medium (IGM), circumgalactic medium (CGM), and interstellar medium (ISM) to the observed Ly alpha damping wing. We generate mock Ly alpha absorption spectra, and consider three idealized scenarios: (i) galaxies embedded in a uniformly ionized IGM, (ii) galaxies surrounded by HII regions with radii of 0-400 pkpc, and (iii) the same models including Ly alpha absorption from partially ionized gas within the virial radius. We explore IGM neutral fractions (x HI) of 0.1, 0.8, and 1.0, and compare the distribution of inferred HI column densities (NHI) with observations to constrain the sizes of HII regions and x HI of the IGM and gas in and around galaxies. Galaxies are surrounded by over-dense gas extending to about 60-100 pkpc, with its extent increasing with halo mass and showing little evolution between z=8 and 10. The inferred NHI also increases with halo and stellar mass. While the observed incidence of damped Ly alpha absorption and the median NHI can be reproduced by different combinations of x HI of the IGM and HII region size, models with a uniformly ionized IGM or HII regions larger than about50 pkpc fail to reproduce the strongest absorbers (NHI> 10 22cm-2). Including partially ionized gas within the virial radius substantially increases the incidence of these absorbers, although the predicted values remain below the observations, suggesting an additional contribution from unresolved gas in stellar birth clouds. The strongest damped Ly alpha absorbers therefore provide a unique probe of the ionization state of the ISM, CGM, and IGM.