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

Abstract

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 $\sim$290-440 pc and resolve the inner $R_{\text{gal}}\lesssim$ 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 (${\Sigma }_{\text{star}}$), the intensity-weighted CO(2-1) ($⟨I_{\text{CO}}⟩$), and the star formation rate surface density (${\Sigma }_{\text{SFR}}$). We find no apparent correlation with ${\Sigma }_{\text{SFR}}$, but positive correlations with the other two parameters, which are stronger in the case of $⟨I_{\text{CO}}⟩$. The HCN/CO-$⟨I_{\text{CO}}⟩$ relations show $\lesssim$0.3 dex galaxy-to-galaxy offsets, with HCN(3-2)/CO(2-1)-$⟨I_{\text{CO}}⟩$ being $\sim$2 times steeper than HCN(1-0)/CO(2-1). In contrast, the HCN(3-2)/HCN(1-0)-$⟨I_{\text{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).

Accepted for publication in A&A. 14 pages, 8 figures

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