3.3. Luminous Infrared Galaxies

The first all-sky, far-infrared survey, conducted in 1983 by the Infrared Astronomical Satellite (IRAS), detected the existence of a large population of galaxies which emit more energy in the infrared (~ 5-500 µm) than at all other wavelengths combined. In the local universe (z 0.3), these luminous infrared galaxies (LIRGs) are the dominant population of sources with luminosities above 10¹¹ L, being more numerous than quasars, Seyfert galaxies, and optically selected starbursts at comparable bolometric luminosity. Morphological studies show that LIRGs are triggered by gas-rich galaxy collisions and mergers. The bulk of the infrared flux is powered by dust heating from a massive starburst within giant molecular clouds. At the highest luminosities, energy input from AGNs becomes important, and LIRGs may represent an important stage in the formation of quasars and powerful radio galaxies. LIRGs may also be an important phase in the formation of elliptical galaxies, globular clusters, and in the metal enrichment of the intergalactic medium. For a recent review of LIRGs, see Sanders & Mirabel (1996).

In terms of studying distant galaxies, the IRAS sample of LIRGs has not provided many sources. The vast majority of IRAS LIRGs are at relatively modest redshift (z 0.4). Only two IRAS sources have been identified at z > 2: IRAS Faint Source 10214+4724 at z = 2.286 (Rowan-Robinson et al. 1991) and the Cloverleaf quasar at z ~ 2.5. Both are gravitationally lensed (e.g., Barvainis et al. 1994; Elston et al. 1994; Graham & Liu 1995; Eisenhardt et al. 1996), with inferred amplification-corrected luminosities similar to the local sample of LIRGs. Studying the unlensed population of these optically obscured sources to higher redshift will be an important step toward understanding the formation and evolution of massive galaxies, but will require more sensitive infrared surveys and/or new search strategies. The Space Infrared Telescope Facility (SIRTF) is expected to identify LIRGs at cosmological distances. Some sub-mm sources are likely to be distant analogs of LIRGs.