Total gas column densities along the X axis within the AB strip under study (top plot) and maximum gas density max(n_H) on the same lines of sight (bottom plot). Shown are the column densities over the full 50 pc lines of sight along the X axis (dashed line) and the column densities for the overdense regions shown in Fig. 3 (solid line). Grey areas mark lines of sight for which less than 50% of the mass is in the overdense region. The dash-dotted lines mark the positions of the maxima of N_H and max(n_H), which are separated by 0.55 pc.