Infrared observations of extragalactic sources

Infrared observations leading to important implications and questions concerning extragalactic sources are reviewed. Attention is given to the detection of enormous infrared fluxes from otherwise normal galactic nuclei and to possible sources, including bursts of star formation, for the infrared luminosity, which has been found to result in mass/luminosity ratios less than 0.2 in solar units. Possible sources for the strong infrared fluxes observed from type 1 and type 2 Seyfert galaxies and nonoptically violently variable quasars are then considered, including thermal reradiation by dust around the nucleus in the case of type 2 Seyferts, and thermal and nonthermal nuclear radiation in association with dust in the case of type 1 Seyferts and quasars. Selection effects favoring the detection of infrared-weak Seyfert galaxies and quasars are also noted