ISO surveys of normal galaxies have already yielded major progress in our grasp of their phenomenology. This progress leads to more informed planning for upcoming surveys or missions, as well as better physical models and understanding of these systems.
Mid-infrared spectral and spatial templates are now available, and the range of variation in these properties is reasonably well characterized. The mid-infrared from the interstellar medium is clearly dominated by emission from particles transiently heated by single-photon events, and most of the emitted energy has the spectral signature of Aromatic compounds. Surveys such as WIRE (Hacking et al. 1999) have already incorporated these data into modeling source signatures, and sorting strategies for identifying the more interesting objects in the 12 and 25µm survey.
Mid-infrared images of galaxies highlight the star forming regions in disk galaxies, but trace the starlight morphology in early-type systems as the interstellar medium drops out. The surface brightness distribution in disk systems could be a major term in detectability with future survey missions at high resolution such as NGST.
The overall spectral energy distribution from 3 to 250µm can now be characterized in a coherent fashion, and its systemtic variations with heating or star forming intensity quantified. There is a definite shift of the luminosity center from 100µm towards 40µm as the intensity increases, and a decrease of the contribution from the mid-infrared and the sub-mm ranges.
The far-infrared cooling lines of the neutral interstellar medium present interesting and complex behavior. Gas cooling via the [CII] line is a smaller fraction of dust cooling in the more active galaxies, whereas the [OI] line seems more constant in relation to dust emission. One obvious implication is that the latter is a better tracer to pursue in high-redshift objects, for which higher luminosity should be associated with higher intensity, and therefore with depressed [CII] emission.
Acknowledgments
ISO is an ESA project with instruments funded by ESA member states, especially France, Germany, the Natherlands and the United Kingdom, and with participation by ISAS and NASA. This work was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract to the National Aeronautics and Space Administraion.