A multi-scale investigation into the diagnostic potential of the HCN/HCO⁺ ratio for active galactic nucleus and starburst activity in nearby galaxies

Context. The identification of active galactic nuclei (AGNs) and starburst (SB) regions in galaxies is crucial for understanding the role of various physical processes in galaxy evolution. Molecular line ratios, such as the HCN/HCO + ratio, have been proposed as potential tracers of these distinct environments. Aims. We aim to assess the reliability of the HCN/HCO + ratio, from J = 1–0 to J = 4–3 transitions, as a diagnostic tool for differentiating AGN and SB activity across a diverse sample of nearby galaxies. We focus on evaluating the effect of spatial resolution on the robustness of these ratios and investigate the underlying physical conditions that drive observed variations. Methods. We compiled observations of HCN and HCO + lines across multiple J transitions from various sources, covering different galaxy types, including Seyferts, SBs, and (ultra-)luminous infrared galaxies. The observations span spatial scales from cloud-sized regions (tens of parsecs) to kiloparsec scales. We analysed the behaviour of these ratios at varying resolutions and employed non-local thermodynamic equilibrium (non-LTE) radiative transfer models to infer the physical conditions that drive the observed ratios. Results. We find that the HCN/HCO + ratio from higher J transitions (e.g. J = 4–3) can differentiate between AGN and SB activity when observed at high spatial resolution (< 100 pc). This distinction occurs around unity, with HCN/HCO < 1 observed in SB-dominated and > 1 in AGN-dominated regions. However, at lower resolutions, contamination from multiple emission sources and beam averaging effects destroy these distinctions. Radiative transfer modelling suggests that elevated HCN/HCO + ratios in AGN-dominated regions are largely driven by an increase in HCN abundance relative to HCO + , likely due to high-temperature chemistry or increased excitation. Conclusions. Our study confirms that the HCN/HCO + ratio, particularly of higher J transitions, can be a reliable tracer of AGN versus SB activity if observations are conducted at sufficiently high spatial resolution. However, caution must be exercised in interpreting these ratios at larger spatial scales due to contamination effects. Further high-resolution observations are needed to refine the conditions under which these ratios can serve as reliable diagnostics.