Ultraluminous infrared galaxies: Far-infrared highlights

We discuss two significant recent developments in the far-IR with relevance to the study of Ultraluminous Infrared Galaxies (ULIRGs). The first of these is the discovery of an isotropic cosmological infrared background radiation (CIB). This has been detected in observations by two different instruments (FIRAS (Puget et al. 1996, Hauser et al. 1998) and DIRBE (Fixsen et al. 1998)) aboard the COBE satellite, with consistent results. The second concerns observations with the ISO satellite at 175 mu m. Several groups have conducted deep surveys with ISO at its most sensitive, long wavelength, 175 mu m. If one assumes that the CIB is a coaddition of discrete sources, it is entirely possible that deep long wavelength observations with ISO can detect these objects (Guiderdoni et al. 1997). Several groups have undertaken such observations (eg. Puget et al. 1998, Kawara et al. 1998), and it would appear that a large number of faint far-IR sources have been found. For the FIRBACK survey, which is discussed in detail, covering about 4 sq. deg. to 175 mu m flux limits of similar to 100mJy, we are detecting similar to 100 sources per sq. deg., which amounts to about 10% of the CIB. The presence of the CIB at these flux levels, and its apparent origin as the integrated flux from numerous point sources, suggests that dust obscuration plays an important role in the early universe. We speculate that the sources contributing to the CIB may be a population of ULIRG-like objects at moderate to high redshift.