  The purpose of this paper is to describe a method for determining a cooling
template for galaxies, using nearby galaxies, and applicable to future
observations of distant galaxies. We observed two starburst galaxies (NGC 253
and Henize 2-10) with the Caltech Submillimeter Observatory in the rotational
lines of carbon monoxide 12CO(J=3-2), (J=6-5) and (J=7-6) for both, and also
12CO(J=4-3) and 13CO(J=3-2) for Henize 2-10 and in the 3P2-3P1 fine-structure
transitions of atomic carbon [CI] at 809 GHz for NGC 253. Some of these
observations have been made previously, but the present multitransition study
(including data found in the literature) is the most complete to date for the
two galaxies. From these observations, we have derived the properties of the
warm and dense molecular gas in the galaxy nuclei. We used an LTE analysis and
an LVG radiative transfer model to determine physical conditions of the
interstellar medium in both sources and predicted integrated line properties of
all CO transitions up to 12CO(15-14). A PDR model has also been used and here
the data are well fitted (within 20 %) by a model cloud with a gas density of
n(H)= 8.0 +- 1.0 * 10E5 cm-3 and an incident FUV flux of chi = 20000 for Henize
2-10. For NGC 253, we deduced n(H) = 3.0 +- 0.5 * 10E5 cm-3 and chi = 20000 for
the modelled cloud. The physical properties of warm gas and CO cooling curves
of the target galaxies are compared with those measured for the nucleus of the
Milky Way and the Cloverleaf QSO. The gas properties and CO cooling curve are
similar for the two starburst galaxies and the Cloverleaf QSO while the Milky
Way nucleus exhibits lower excitation molecular gas.
