Comparison of N₂H⁺ and HCN emission. (a) Pixel-by-pixel distribution of integrated N₂H⁺ and HCN emission in linear (left panel) and logarithmic (right panel) scaling. Subsets of pixels are visually isolated based on their high N₂H⁺ (pink, SW.Arm) or HCN (yellow, AGN) values. Their location relative to the distribution of HCN emission in M 51 is shown in b) (contour marks 5σ N₂H⁺ integrated intensity). Power-law fits are applied to all data points (black dashed line), the subsets identified (solid lines), as well as all data points excluding the yellow (AGN-affected) subset (black dotted line). Fit parameters and Spearman correlation coefficients for all data points and the subsets are given in Table 2. Data points below the 3σ noise level are presented as grey crosses. The N₂H⁺-to-HCN relation from Jiménez-Donaire et al. (2023) is shown as a green dotted line.