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