2023A&A...672A..96G

We present new HCN and HCO⁺ (J = 3-2) images of the nearby star-forming galaxies (SFGs) NGC 3351, NGC 3627, and NGC 4321. The observations, obtained with the Morita ALMA Compact Array, have a spatial resolution of ∼290-440 pc and resolve the inner R_gal ≲ 0.6-1 kpc of the targets, as well as the southern bar end of NGC 3627. We complement this data set with publicly available images of lower excitation lines of HCN, HCO⁺, and CO and analyse the behaviour of a representative set of line ratios: HCN(3-2)/HCN(1-0), HCN(3-2)/HCO⁺(3-2), HCN(1-0)/CO(2-1), and HCN(3-2)/CO(2-1). Most of these ratios peak at the galaxy centres and decrease outwards. We compare the HCN and HCO⁺ observations with a grid of one-phase, non-local thermodynamic equilibrium (non-LTE) radiative transfer models and find them compatible with models that predict subthermally excited and optically thick lines. We study the systematic variations of the line ratios across the targets as a function of the stellar surface density (Σ_star), the intensity-weighted CO(2-1) (⟨I_CO⟩), and the star formation rate surface density (Σ_SFR). We find no apparent correlation with Σ_SFR, but positive correlations with the other two parameters, which are stronger in the case of ⟨I_CO⟩. The HCN/CO-⟨I_CO⟩ relations show ≲0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-⟨I_CO⟩ being ∼2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-⟨I_CO⟩ relation exhibits a tighter alignment between galaxies. We conclude that the overall behaviour of the line ratios cannot be ascribed to variations in a single excitation parameter (e.g., density or temperature).