  The submillimeter CO lines are formed in warm and dense molecular gas and are
therefore sensitive to the physical conditions whereas the CO (J=1-0) line is a
tracer of the total molecular gas mass. In order to be able to compare the
properties of molecular gas in nearby and distant galaxies, we have observed C
and CO submillimeter lines (including the 12CO(6-5) and 12CO(7-6) lines) in a
sample of nearby galaxies using the CSO. We have obtained a complete view of
the CO cooling curve (also called CO spectral energy distribution) by combining
the submillimeter CSO data with previous observations found in the literature.
We made use of Large Velocity Gradient (LVG) models to analyse the observed CO
cooling curve, predict CO line intensities from J=1-0 to J=15-14 in the studied
galaxies, and derive the physical properties of the warm and dense molecular
gas : the kinetic temperature (TK); the gas density (n(H2)); the
N(12CO)/$\Delta v$. The predictions for the line intensities and for the total
CO cooling power, obtained from LVG modelling have been compared with
predictions from Photo Dissociation Regions (PDR) models. We show how the CO
SED varies according to the galaxy star forming activity. For active nuclei,
the peak is located near the 12CO(6-5) or 12CO(7-6) rotational lines, while,
for normal nuclei, most of the energy is carried by the 12CO(4-3) and 12CO(5-4)
lines. Whatever the spectral type of the nucleus, the observed C cooling rate
is lower than the observed CO cooling rate (by a factor of around 4). The CO
cooling curve of nearby starburst galaxies (e.g. NGC 253) has a quite similar
shape to the CO cooling curve of distant galaxies. Therefore, the CO cooling
curves are useful diagnostics for the star forming activity in distant objects.
