Abstract
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 (nH2 > 106 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. Further, the starburst galaxies NGC 253 and M82 have a much higher fraction of molecular mass at high density (nH2 > 104 cm-3) than the normal galaxies IC 342 and the Milky Way. These results imply that the clouds in starburst nuclei form stars more efficiently than those in normal galaxies.
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