The Average Properties of the Dense Molecular Gas in Galaxies

摘要

We have observed the HCN J = 3 - 2 and J = 1 - 0 emission from several nearby starburst and normal galaxies. These lines have large critical densities (n_H_2 > 10~6 cm~-3) and excitation energies (Eu > 25 K).Thus, they probe the warm and dense molecular gas where massive stars typically form. The average cloud densities of these galaxies, as estimated from the ratio of the HCN J = 3 - 2 and 1 - 0 integrated intensities, is correlated with their star formation efficiency. Therefore, the average densities of the molecular clouds in starburst nuclei are higher than those of more quiescent galaxies.