Ultraluminous Infrared Galaxies: Far-Infrared Highlights

Abstract

We discuss two significant recent developments in the far-IRwith relevance to the study of Ultraluminous Infrared Galaxies(ULIRGs). The first of these is the discovery of an isotropiccosmological infrared background radiation (CIB). This has beendetected in observations by two different instruments (FIRAS (Puget etal. 1996, Hauser et al. 1998) and DIRBE (Fixsen et al. 1998)) aboardthe COBE satellite, with consistent results. The second concernsobservations with the ISO satellite at 175μm. Several groups haveconducted deep surveys with ISO at its most sensitive, longwavelength, 175μm. If one assumes that the CIB is a coaddition ofdiscrete sources, it is entirely possible that deep long wavelengthobservations with ISO can detect these objects (Guiderdoni et al.1997). Several groups have undertaken such observations (eg. Puget etal. 1998, Kawara et al. 1998), and it would appear that a large numberof faint far-IR sources have been found. For the FIRBACK survey, whichis discussed in detail, covering about 4 sq. deg. to 175μm fluxlimits of ∼100mJy, we are detecting ∼ 100 sources persq. deg., which amounts to about 10% of the CIB. The presence of theCIB at these flux levels, and its apparent origin as the integratedflux from numerous point sources, suggests that dust obscuration playsan important role in the early universe. We speculate that the sourcescontributing to the CIB may be a population of ULIRG-like objects atmoderate to high redshift.