, D. Downes 
, S.J.E. Radford 
,
J.W. Barrett Astronomy Program, State University of New York, Stony Brook,
NY 11794,USA Institut de Radio Astronomie Millimétrique,
38406 St. Martin d'Hères, France National Radio Astronomy Observatory, Tucson AZ 85721, USA
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.