We present observations with the IRAM 30 m telescope of CO in a large sample of ultraluminous IR galaxies out to redshift z = 0.3. Most of the ultraluminous galaxies in this sample are interacting, but not completed mergers. The CO(1-0) luminosity of all but one of the ultraluminous galaxies is high, with values of . The extremely small dispersion of only 30 % is less than that of the far infrared luminosity. The integrated CO line intensity is strongly correlated with the m flux density, as expected for a black body model in which the mid and far IR radiation is optically thick. We use this model to derive sizes of the FIR and CO emitting regions and the enclosed dynamical masses. Both the IR and CO emission originate in regions a few hundred parsecs in radius. The median value of , within a factor of two or three of the black body limit for the observed far IR temperatures. The entire ISM is a scaled up version of a normal galactic disk with the ambient densities a factor of 100 higher, making even the intercloud medium a molecular region. We compare three different techniques of H mass estimation and conclude that the ratio of gas mass to CO luminosity is about a factor of four times lower than for Galactic molecular clouds, but that the gas mass is a large fraction of the dynamical mass. Our analysis of CO emission from ultraluminous galaxies reduces the H mass from previous estimates of to , which is in the range found for molecular gas rich spiral galaxies. A collision involving a molecular gas rich spiral could lead to an ultraluminous galaxy powered by central star bursts triggered by the compression of infalling preexisting GMC's.
The extremely dense molecular gas in the center of an ultraluminous galaxy is an ideal stellar nursery for a huge star burst.