  We study the relationship between gas cooling via the [C II] (158 micron)
line emission and dust cooling via the far-IR continuum emission on the global
scale of a galaxy in normal (i.e. non-AGN dominated and non-starburst)
late-type systems. It is known that the luminosity ratio of total gas and dust
cooling, L(C II)/L(FIR), shows a non-linear behaviour with the equivalent width
of the Halpha line emission, the ratio decreasing in galaxies of lower massive
star-formation activity. This result holds despite the fact that known
individual Galactic and extragalactic sources of the [C II] line emission show
different [C II] line-to-far-IR continuum emission ratios. This non-linear
behaviour is reproduced by a simple quantitative model of gas and dust heating
from different stellar populations, assuming that the photoelectric effect on
dust, induced by far-UV photons, is the dominant mechanism of gas heating in
the general diffuse interstellar medium of the galaxies under investigation.
According to the model, the global L(C II)/L(FIR) provides a direct measure of
the fractional amount of non-ionizing UV light in the interstellar radiation
field and not of the efficiency of the photoelectric heating. The model also
defines a method to constrain the stellar initial mass function from
measurements of L(C II) and L(FIR). A sample of 20 Virgo cluster galaxies
observed in the [C II] line with the LWS on board ISO is used to illustrate the
model. The limited statistics and the necessary assumptions behind the
determination of the global [C II] luminosities from the spatially limited data
do not allow us to establish definitive conclusions but data-sets available in
the future will allow tests of both the reliability of the assumptions of our
model and the statistical significance of our results.
