A survey for extragalactic HCN and HCO(+)

We have surveyed 15 galaxies for HCN and HCO^+^ J = 1 - 0 emission and detected HCN in 12 objects and HCO^+^ in 11. Maps were made of IC 342, M51, and M83. Since the HCN and HCO^+^ lines require higher densities than CO J = 1 - 0 for excitation, they probe denser gas. Strong HCO^+^ J = 3 - 2 emission has been found in NGC 253. Our new J = 1 - 0 detections along with the J = 3 - 2 data are used to estimate excitation temperatures and optical depths. We find T_ex_~20 K for NGC 253 and T_ex_ ~10 K for M82 and IC 342, which indicates a substantially larger kinetic temperature and/or volume density in NGC 253. The HCN/HCO^+^ line ratio varies from 0.5 to >~ 4, independent of obvious star formation indicators. In M51 the inner spiral arms traced by CO are detected in HCN. In this galaxy the HCN emission peak does not coincide with the strongest CO peak but is instead located at the center of the nuclear CO ring. The maps of HCN and HCO^+^ J = 1 - 0 emission toward IC 342 are significantly different, especially at the periphery of the nuclear region. These results show that CO, HCN, and HCO^+^ probe distinct physical regions. The HCN and HCO^+^ J= 1-0 emission is only weakly correlated with CO J= 1-0 emission in a similar beam, suggesting that either the dense gas content or the HCN and HCO^+^ abundances vary from galaxy to galaxy. In starburst and Seyfert galaxies, the emission of HCO^+^ tends to be more tightly correlated with that of CO and with the nonthermal continuum emission of the central compact radio source. This indicates that the production and/or excitation of HCO^+^ may be due to increased ionization by cosmic rays in the nuclear star-forming regions. The H^12^CN/H^13^CN and H^12^CO^+^/H^13^CO^+^ intensity ratios in IC 342, NGC 253, and M82 are comparable to the ^12^CO/^13^CO ratios. This result is consistent with chemical models in the case of low electron density and weak line saturation.