Sub-kiloparsec empirical relations and excitation conditions of HCN and HCO⁺ <i>J</i> = 3–2 in nearby star-forming galaxies

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