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 local stellar mass surface density. 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_⋆). 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). The green diamonds connected by a solid line are the running medians from Bigiel et al. (2016) from their single-dish map of the entire M 51 disc (28″ ∼ 1 kpc resolution). The dashed black lines are the best-fit regressions from Usero et al. (2015), for single-dish HCN observations of nearby galaxies at kpc-scale resolution. The dashed-dotted black lines are the fits to the ALMA observations presented in Gallagher et al. (2018a) for four nearby galaxies, with a synthesised beam of a few hundred parsec sampled in radial bins.