Dense molecular clouds and the ARP 220 starburst

Using the IRAM 30 m telescope we have detected CS(3 -> 2) and ^13^CO(2 -> 1) emission from Arp 220, the prototype infrared luminous galaxy. The CS luminosity of Arp 220 is half the ^12^CO luminosity of the Milky Way. Since CS emission traces gas 100 times denser than that traced by CO emission, Arp 220 contains much more molecular gas at high densities than normal galaxies. We estimate there are about 10^10^ M_sun_ of molecular gas at the high densities [n(H_2_) = 10^5^ cm^-3^] characteristic of star-forming molecular cloud cores. The existence of these clouds supports a starburst rather than an accretion disk-black hole origin for the 10^12^ L_sun_ emitted in the far-infrared. The 10^10^ M_sun_ of dense gas is a vital intermediary in the process of star formation and galactic evolution. The important molecular indicator which differentiates infrared luminous galaxies from ordinary spirals may therefore be CS rather than CO emission.