The origin and evolution of ULIGs continues to be a subject of intense research, and one that has taken on renewed importance following new infrared/submillimeter results from ISO and SCUBA/JCMT, and from new X-ray observations with ROSAT, ASCA, and BeppoSax. Strong evolution in the space density of luminous infrared galaxies has been detected in the first deep mid-infrared surveys with ISOCAM (Taniguchi et al. 1997; Aussel et al. 1999) and deep far-infrared surveys with ISOPHOT (Kawara et al. 1998; Puget et al. 1999). Reports from the first deep submillimeter surveys with SCUBA on the JCMT (Smail et al. 1997; Hughes et al. 1998; Barger et al. 1998; Eales et al. 1999) show that the space density of ULIGs at high redshift (z > 1) appears be sufficient to account for nearly all of the far-infrared/submillimeter background radiation (e.g. Barger, Cowie, & Sanders 1999), and depending on their exact redshift distribution, produces an infrared luminosity density that exceeds that in the optical/UV by factors of 2-5 (e.g. models by Blain et al. 1999). In addition, the discovery that much of the X-ray background appears to be produced by a population of heavily obscured AGN (e.g. Fabian & Barcons 1992; Boyle et al. 1995; Almaini et al. 1998), objects which have been largely missed in optical surveys due to extremely heavy obscuration along the line of sight (e.g. NH 1024-25 cm-2: Maiolino et al. 1998) has clearly renewed interest in studies of infrared-selected AGN.
It seems clear that detailed observations of nearby ULIGs are a required first step in order to better understand these objects, and in particular to unravel the nature of the dominant energy source responsible for their enormous infrared luminosity. Given the evidence from millimeterwave interferometer observations which indicate large absorbing columns toward the nuclei of all ULIGs [typically N(H2) > 1024 cm-2: e.g. Bryant & Scoville 1996), it has always seemed prudent to use a broad multiwavelength approach to study these sources, thus we welcome the new mid- and far-infrared ISO data as the latest tool in the study of ULIGs.
The four topics posed for the debate are answered in the order they were posed.