A survey for extragalactic HCN and HCO(+)

Abstract

Fifteen galaxies were surveyed for HCN and HCO(+)J = 1 - 0 emission; HCN was detected in 12 objects and HCO(+) in 11. Maps were made of IC 342, M51, and M83. Strong HCO(+)J = 3 - 2 emission was found in NGC 253. New J = 1 - 0 detections along with the J = 3 - 2 data are used to estimate excitation temperatures and optical depths. The HCN/HCO(+) line ratio varies from 0.5 to greater than about 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. 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.