Star formation efficiency of the dense gas (SFE_dense = Σ_SFR/Σ_dense, left) and dense gas fraction (F_dense = Σ_dense/Σ_mol, right) as a function of the local velocity dispersion of HCN. The filled circles show results for 3″ apertures (∼100 pc) centred on HCN peaks, while the small open circles in the background represent all detections across our fields of view (above 3σ simultaneously for SFE_dense, F_dense, and σ_HCN). The right panel shows more detections because the signal-to-noise ratio in CO and HCN is high compared to 33 GHz (limiting the detections on the left panel). Open circles with a downward arrow indicate upper limits. The blue triangles and gray squares correspond to measurements at comparable scales for NGC 3627 (Murphy et al. 2015) and M 31 (Brouillet et al. 2005, and new SFRs described in the text).