Distribution of the atomic hydrogen column density integrated over 50 pc from each star particle as a function of stellar age. We measure the column density around star particles within 0.2 r_vir in all snapshots. The column density is averaged over six directions, and the stellar age is uniformly binned on a logarithmic scale into 8 intervals from 0.1 to 100 Myr. The mean logarithmic column densities of each bin are indicated by crosses. Note that the column density decreases faster in the SINK-Fid run.