IRAS Observations of Galaxies

The basic properties of galaxies at 12μm, 25μm, 60μm and 100μm have been determined from IRAS observations. The far infrared luminosity of galaxies is a substantial, but not dominant, component of the luminosity output of the local universe. From a careful analysis of the spatial distribution of the infrared emission in the nearby spiral galaxy M33 it is found that approximately 3/4 of the total infrared luminosity from this galaxy originates in stars with ages less than a few hundred million years. To the extent that M33 is a typical spiral galaxy, this supports the prejudice that the infrared emission in galaxies is measuring the luminosity of newly formed stars. Interactions are found to be a major trigger of the high activity phase (L_(ir) > 10¹¹ L_⊙) in galaxies. A substantial fraction of all massive spiral galaxies probably undergo such a phase over the course of their evolution