The distribution of Lyman-limit absorption systems during and after reionization

Abstract

Lyman-limit absorption systems can play many important roles during and after cosmological reionization. Unfortunately, due to the prohibitively large dynamic range required, it is impossible to self-consistently include these systems in cosmological simulations. Using fast and versatile semi-numeric simulations, we systematically explore the spatial distribution of absorption systems during and following reionization. We self-calibrate the resulting number of absorbers to the mean free path (mfp) of the ionizing ultraviolet background (UVB), and present results at a given mfp and neutral hydrogen fraction. We use a simple optical depth criterion to identify the locations of absorbers. Our approach is fairly robust to uncertainties such as missing subgrid structure. Unlike at lower redshifts where the UVB is relatively uniform, at higher redshifts the fluctuations in the UVB and the HII morphology of reionization can drive the large-scale distribution of absorption systems. Specifically, we find that absorbers are highly correlated with the density field on small scales, and then become anti-correlated with the UVB on large scales. After reionization, the large-scale power spectrum of the absorbers traces the UVB power spectrum, which can be predicted with a simple analytic extension of the halo-model. During reionization, absorbers tend to preferentially lie inside overdensities (i.e. filaments) of the recently-ionized intergalactic medium (IGM). Absorbers may also dominate the small-scale (k > 1/Mpc) 21-cm power during and after reionization. Conversely, they smooth the contrast on moderate scales. Once the HII regions grow to surpass the mfp, the absorbers add to the large-scale 21-cm power. Our results should prove useful in interpreting future observations of the reionization epoch.