2022ApJ...936...58Z

Both Galactic and extragalactic studies of star formation suggest that stars form directly from dense molecular gas. To trace such high volume density gas, HCN and HCO⁺J = 1 → 0 have been widely used for their high dipole moments, relatively high abundances, and often being the strongest lines after CO. However, HCN and HCO⁺J = 1 → 0 emission could arguably be dominated by the gas components at low volume densities. The HCN J = 2 → 1 and HCO⁺J = 2 → 1 transitions, with more suitable critical densities (1.6 × 10⁶ and 2.8 × 10⁵ cm^–3) and excitation requirements, would trace typical dense gas closely related to star formation. Here we report new observations of HCN J = 2 → 1 and HCO⁺J = 2 → 1 toward 17 nearby infrared-bright galaxies with the APEX 12 m telescope. The correlation slopes between the luminosities of HCN J = 2 → 1 and HCO⁺J = 2 → 1 and total infrared emission are 1.03 ± 0.05 and 1.00 ± 0.05, respectively. The correlations of their surface densities, normalized with the area of radio/submillimeter continuum, show even tighter relations (slopes: 0.99 ± 0.03 and 1.02 ± 0.03). The eight active galactic nucleus (AGN)-dominated galaxies show no significant difference from the 11 star-formation-dominated galaxies in the above relations. The average HCN/HCO⁺ ratios are 1.15 ± 0.26 and 0.98 ± 0.42 for AGN- and star-formation-dominated galaxies, respectively, without obvious dependencies on infrared luminosity, dust temperature, or infrared pumping. The Magellanic Clouds roughly follow the same correlations, expanding to 8 orders of magnitude. On the other hand, ultraluminous infrared galaxies with AGNs systematically lie above the correlations, indicating potential biases introduced by AGNs.