Annu. Rev. Astron. Astrophys. 2005. 43: xxx-xxx Copyright © 2005 by Annual Reviews. All rights reserved

2.3. Local Infrared Galaxies

A few very luminous infrared galaxies were observed in the seventies (Rieke & Lebofsky 1979). Then IRAS satellite, launched in 1983 gave for the first time a proper census of the infrared emission of galaxies at low redshift. The Luminosity Function (LF) at 60 and 100 µm is dominated by L spiral galaxies as could be expected - the reradiated stellar luminosity absorbed by dust. In addition, a high-luminosity tail of luminous galaxies was found (e.g., Sanders & Mirabel 1996). This high-luminosity tail can be approximated by a power-law, (L) L_IR^2.35, which gives a space density for the most luminous infrared sources well in excess of predictions based on the optical LF. These sources comprise the Luminous Infrared Galaxies, LIRGs, and the ULIRGs with luminosities 11 < log(L_IR / L) < 12 and log(L_IR / L) > 12, respectively. These galaxies are often associated with interacting or merging, gas-rich disks. The fraction of strongly interacting/merger systems increases from ~ 10% at log(L_IR / L) = 10.5-11 to ~ 100% at log(L_IR / L) > 12. LIRGs are the site of intense starburst activity (about 10-100 M year^-1) induced by the interaction and/or strong spiral structure. The ULIRG phase occurs near the end of the merging process when the two disks overlap. Such galaxies may be the precursors of Quasi Stellar Objects (QSOs; Sanders et al. 1988a, 1988b; Veilleux et al. 1995; Lutz et al. 1999). These objects have been the subject of intense debate concerning the nature of the dominant source of emission: starburst versus dust-enshrouded AGN (e.g., Filipenko 1992; Sanders & Mirabel 1996; Joseph 1999). Indeed, spectra show evidence of extremely large optical depth (heavily reddened continuum and large Balmer decrement) but also exhibit AGN-like high excitation fine-structure lines. We had to wait for ISO to clearly determine the power sources of ULIRGs. The difference between the mid-infrared spectra of starburst and AGNs is striking. Starburst are often characterized by strong, low-excitation, fine-structure lines, prominent PAH features and a weak 10 µm continuum whereas AGNs display a highly excited emission line spectrum with weak or no PAH features, plus a strong mid-infrared continuum. It has been thus possible to build mid-infrared diagnostic diagrams (e.g., Genzel et al. 1998; Laurent et al. 2000) that clearly separates starburst-dominated galaxies from AGN-dominated galaxies. These diagrams demonstrate that ULIRGs appear to be composite objects, but star formation dominates in most objects. That is on average, 70% of the reradiated energy comes from starbursts and 30% comes from AGNs (Genzel et al. 1998; Lutz et al. 1998). However the fraction of AGN-powered objects increases with luminosity. About 15% of ULIRGs at luminosities below 2 × 10¹² L are mostly AGN powered, but this fraction increases to about half at higher luminosity.

All these well-studied LIRGs and ULIRGs are at low redshift. They do not dominate the energy production locally. As an example, the total infrared luminosity from these galaxies in the IRAS Bright Galaxy Sample accounts for only ~ 6% of the infrared emission in the local Universe (Soifer & Neugeubauer, 1991). As we will see, the situation changes dramatically at higher redshift where these galaxies fully dominate the infrared energy output.