We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the Hi column density distribution f (N HI) of damped Lyα systems (DLAs) and sub-DLAs at z = 3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the HI cloud too deeply, and therefore we underestimated the f (NH I) at 19 < logNH I < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n gas > 6 × 10-3 cm-3, then we reproduce the observed f (NH I) at z = 3 very well. We also investigate the effect of local stellar radiation by postprocessing our simulation with a radiative transfer code and find that the local stellar radiation does not change the f (NH I) very much. Our results show that the shape of f (NH I) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically thick neutral gas to optically thin ionized gas within dark matter halos.
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