[S II]6716,31/[O III]4959,5007 ratio map (reddening corrected), tracing the ionisation structure of the gas. The cyan contours indicate a ratio of 0.5, which we use as fiducial limit for the transition to optically thick gas. Numbered black contours correspond to the H II regions analysed in Sect. 7. The position of WR stars (yellow crosses) and planetary nebulae (white plus symbols) is also indicated.