HCN spectroscopic ratios against molecular cloud properties. Top: HCN/CO and (bottom) SFR/HCN measured at 260 pc scale versus cloud-scale molecular gas properties inferred from 120 pc scale CO(2–1) observations. Panels from left to right show molecular cloud surface density (Σ_mol), velocity dispersion (σ_mol) and virial parameter (α_vir) on the x-axis, obtained from PHANGS–ALMA as described in Sect. D.1. Similarly to Fig. 7 and 8, markers indicate the respective environments and line fits as well as linear regression regimes are determined via MARS. The obtained thresholds are ${\mathrm{\Sigma }}_{\text{mol}}=74{\mathrm{M}}_{\odot }{\mathrm{pc}}^{-2}$ for SFR/HCN versus Σ_mol and σ_mol = 8.3 km s⁻¹ for SFR/HCN versus σ_mol. The linear regression parameters are listed in Tab. D.1.