Average HCN/CO line ratio as function of Σ_mol, with Σ_mol being calculated with four different methods: a) a constant α_CO conversion factor multiplied to ¹²CO emission (Σ_mol^const) b) the measured α_CO as described in Sect. 2.3.2, Σ_mol^measured c) a metallicity and stellar mass based α_CO, ${\mathrm{\Sigma }}_{\mathrm{mol}}^{\mathrm{metal},*}$, and an independent dust-based estimate of molecular gas surface density from Faustino Vieira et al. (2024), ${\mathrm{\Sigma }}_{\mathrm{mol}}^{\mathit{dust}}$.