The impact of local stellar radiation on the H I column density distribution

It is often assumed that local sources of ionizing radiation have little impact on the distribution of neutral hydrogen in the post-reionization Universe. While this is a good assumption for the intergalactic medium, analytic arguments suggest that local sources may typically be more important than the metagalactic background radiation for high column density absorbers (N-HI > 10(17) cm(-2)). We post-process cosmological, hydrodynamical simulations with accurate radiation transport to investigate the impact of local stellar sources on the column density distribution of neutral hydrogen. We demonstrate that the limited numerical resolution and the simplified treatment of the interstellar medium (ISM) that are typical of the current generation of cosmological simulations provide significant challenges, but that many of the problems can be overcome by taking two steps. First, using ISM particles rather than stellar particles as sources results in a much better sampling of the source distribution, effectively mimicking higher resolution simulations. Secondly, by rescaling the source luminosities so that the amount of radiation escaping into the intergalactic medium agrees with that required to produce the observed background radiation, many of the results become insensitive to errors in the predicted fraction of the radiation that escapes the immediate vicinity of the sources. By adopting this strategy and by drastically varying the assumptions about the structure of the unresolved ISM, we conclude that we can robustly estimate the effect of local sources for column densities N-HI << 10(21) cm(-2). However, neither the escape fraction of ionizing radiation nor the effect of local sources on the abundance of N-HI greater than or similar to 10(21) cm(-2) systems can be predicted with confidence. We find that local stellar radiation is unimportant for N-HI << 10(17) cm(-2), but that it can affect Lyman Limit and Damped Ly alpha systems. For 10(18) < N-HI < 10(21) cm(-2) the impact of local sources increases with redshift. At z = 5 the abundance of absorbers is substantially reduced for N-HI >> 10(17) cm(-2), but at z = 0 the effect only becomes significant for N-HI << 10(21) cm(-2).