N_H I and CO line-integrated intensity map. Left: H I column density map determined from VLA H I observations (Gratier et al. 2010). Right: IRAM 30 m CO(2 − 1) line integrated intensity map of M33 (Druard et al. 2014). The lines in the colour bar mark the 3σ and 6σ values of CO emission. Both maps are smoothed to a resolution of 18.2″ and re-gridded to a pixel size of 6″. CO contours at 3 and 6σ are overlaid on both maps.