ISO has not only advanced the knowledge of IR properties of normal galaxies but has also made unexpected discoveries. The bulk of the emission from dust has been measured, revealing cold dust in copious quantities. This dust is present in all types of normal galaxies and is predominantly distributed in a diffuse disk with an intrinsic scalelength exceeding that of the stars. Cold dust has been found beyond the optical regions of isolated galaxies, associated with the extended HI disks of spiral galaxies or with the HI envelopes of dwarf galaxies. The fraction of the bolometric luminosity radiated by dust has been measured for the first time. Realistic geometries for stars and dust have been derived from ISO imaging observations, enabling the contribution of the various stellar populations to the dust heating to be accurately derived. The NIR/MIR SEDs were shown to be remarkably similar for normal spiral galaxies and to originate mostly in the PDRs surrounding star-forming regions. A diffuse component of the MIR emission was found, though puzzlingly with a much smaller scalelength than its FIR counterpart. The importance of the central regions in shaping up the intensity and the colour of the MIR global emission was revealed thanks to the high spatial resolution offered by ISO. A new component of interstellar dust emission consisting of a "hot" NIR continuum emission was discovered. The relative contribution of photospheric and very small grains/ PAH emission has been established in the NIR/MIR spectral region. Spectroscopic observations have revealed that the main interstellar cooling line, [CII], is predominantly carried by the diffuse cold neutral medium in normal galaxies, with emission from localised PDRs only dominating for galaxies having high star-formation activity.
This enormous advancement in the understanding of normal galaxies in the nearby Universe has laid the foundation for more detailed investigations with Spitzer and Herschel. A clear priority is to fill the gap left by ISO in knowledge of the SEDs of normal galaxies between 20-60 µm. Another priority is to increase the number of galaxies with detailed imaging information and to provide better statistics on carefully selected samples, especially those selected in the optical/NIR bands. Ultimately, the improved sensitivity of the new infrared space observatories will allow knowledge of the dust emission from normal galaxies to be extended beyond the nearby universe.
Acknowledgements
The authors would like to take this opportunity to thank all the individuals that helped make the ISO mission a success. M. Sauvage acknowledges the Max Planck Institut für Kernphysik for its support during the final adjustments of the manuscript. R.J. Tuffs and C.C. Popescu would also like to hank Heinrich J. Völk for enlightening discussions.