Correlation between N₂H⁺ (1-0), HCN (1-0) and CO (1-0) intensities and line ratios. Left: comparison between I_N2H⁺ and I_HCN in our observed locations (yellow stars), including available data for nearby galaxies (circles, Aladro et al. 2015; Watanabe et al. 2014; Mauersberger & Henkel 1991; Nguyen et al. 1992; Eibensteiner et al. 2022; Nishimura et al. 2016b; Takano et al. 2019) and Galactic molecular clouds (squares, Jones et al. 2012; Pety et al. 2017; Barnes et al. 2020; Yun et al. 2021). The solid line represents a linear fit to the ensemble data in both panels, showing that the two emission lines are strongly correlated (r > 0.98) across almost four orders of magnitude. The dashed lines indicate a 3σ dispersion from the mean. Right: I_N2H⁺/I_CO as a function of I_HCN/I_CO for NGC 6946 and the same literature compilation. The two ratios, both proxies for the dense gas fraction, also remain well correlated across almost three orders of magnitude.