Gas cooling within the diffuse ISM of late-type galaxies

We combine observations of spiral galaxies in the [CII] line at 158 mum, ma de with the Long Wavelength Spectrometer aboard ISO, with previous data fro m the Kuiper Airborne Observatory to study the origin of this line, which i s the main coolant of the interstellar medium at relatively low temperature s. We also use HI and CO(1-0) observations of these galaxies and estimate t he respective line fluxes in the same beam as the [CII] observations. We co nfirm the existence of a linear relation between the [CII] line intensity a nd the CO(1-0) line intensity, that we extend to intrinsically fainter gala xies. The dispersion around this relation is significant and due to variati ons in the far-UV flux, thus in the star formation rate. We find that for t he least active galaxies of our sample, in terms of star formation, the rat e of [CII] line emission per interstellar hydrogen atom is similar to that in the Solar neighbourhood. For those galaxies, most of the [CII] line emis sion comes probably from the diffuse cold atomic medium. In more active gal axies, considered globally, the average [CII] line emission is dominated by dense photodissociation regions and to some extent by the warm ionized dif fuse medium. This is true in the central regions of many spiral galaxies, a nd probably even in the interarm regions of the most actively star-forming ones.