Annu. Rev. Astron. Astrophys. 2000. 38: 761-814 Copyright © 2000 by Annual Reviews. All rights reserved

3.4. The Nature of Ultra-Luminous Infrared Galaxies

ULIRGs [L(8-1000 µm) 10¹² L; Soifer et al 1984, 1987] are mergers of gas-rich disk galaxies (cf Sanders & Mirabel 1996, Moorwood 1996). Luminosities and space densities of ULIRGs in the local Universe are similar to those of QSOs (Soifer et al 1987, Sanders & Mirabel 1996). In a classical paper Sanders et al (1988a) proposed that most ULIRGs are predominantly powered by dust-enshrouded QSOs in the late phases of a merger. The final state of a ULIRG merger may be a large elliptical galaxy with a massive quiescent black hole at its center (Kormendy & Sanders 1992). Despite a host of observations during the last decade, the central questions regarding what, on average, dominates the luminosity of ULIRGs, and how they evolve, are by no means answered. On the one hand, their IR, mm, and radio characteristics are similar to those of starburst galaxies (Rieke et al 1985, Rowan-Robinson & Crawford 1989, Condon et al 1991b, Sopp & Alexander 1991, Rigopoulou et al 1996b, Goldader et al 1995, 1997a, b, Acosta-Pulido et al 1996, Klaas et al 1999). Particularly compelling is the detection of a number of compact radio hypernovae in each of the two nuclei of Arp 220 (Smith et al 1998). The extended optical emission line nebulae resemble the expanding "superwind bubbles" of starburst galaxies (Armus et al 1990, Heckman et al 1990). On the other hand, a significant fraction of ULIRGs exhibits nuclear optical emission line spectra characteristic of Seyfert galaxies (Sanders et al 1988a, Armus et al 1990, Kim et al 1995, 1998, Veilleux et al 1995, 1997, 1999). Some contain compact central radio sources (Lonsdale et al 1993, 1995) and highly absorbed, hard X-ray sources (Mitsuda 1995, Brandt et al 1997, Kii et al 1997, Vignati et al 1999), all indicative of an active nucleus (AGN). With the advent of ISO, sensitive mid- and far-IR spectroscopy have become available and have allowed a fresh look at the issues of the energetics, dynamics, and evolution of ULIRGs.