Observations with the Infrared Astronomical Satellite (IRAS) led to the identification of many highly luminous galaxies that emit a substantial fraction of their energy in the far infrared, i.e., at > 10 µm (Soifer, Houck, and Neugebauer 1987), although the existence of such luminous far-IR sources had been known previously (e.g., Rieke and Low 1972). Ultraluminous far-infrared galaxies have L(8-1000 µm) 1012 L and have far-infrared luminosities that exceed their optical luminosities by a factor of 10 or more. Many extragalactic IRAS sources are starburst galaxies. Some IRAS sources are identified with known AGNs, and in other cases the IRAS observations led to identification of AGNs not previously known. The far-infrared emission in these sources is thermal radiation from dust (at T 100 K or less) that is heated either by massive star formation or by a ``hidden'' AGN which we do not observe directly through the dust.
Adapted from B.M. Peterson An Introduction to Active Galactic Nuclei, Cambridge University Press, (1997)