Normal Nearby Galaxies

Following on from IRAS, ISO has provided a huge advancement in our knowledge of the phenomenology of the infrared (IR) emission of normal galaxies and the underlying physical processes. Highlights include the discovery of an extended cold dust emission component, present in all types of gas-rich galaxies and carrying the bulk of the dust luminosity; the definitive characterisation of the spectral energy distribution in the IR, revealing the channels through which stars power the IR light; the derivation of realistic geometries for stars and dust from ISO imaging; the discovery of cold dust associated with H I extending beyond the optical body of galaxies; the remarkable similarity of the near-IR (NIR)/mid-IR (MIR) SEDs for spiral galaxies, revealing the importance of the photo-dissociation regions in the energy budget for that wavelength range; the importance of the emission from the central regions in shaping up the intensity and the colour of the global MIR luminosity; the discovery of the “hot” NIR continuum emission component of interstellar dust; the predominance of the diffuse cold neutral medium as the origin for the main interstellar cooling line, [C II] 158 μm, in normal galaxies.