Maps of the oxygen column density (top panel) and molecular hydrogen volume density (middle panel) over the shocked region where OI emission is detected. The bottom panel shows a map of the geometrical thickness along the line of sight of the same region, derived from the ratio between N_O and , assuming a constant oxygen abundance relative to molecular hydrogen of 6 × 10^–4. The contours represent the same map of the gas column density as in Fig. 8.