4.4. High Redshift Luminous Infrared Galaxies
A significant galaxy population at high redshift is the luminous infrared galaxies detected by redshifted mid-infrared emission from dust grains at observed wavelengths 10 - 1000 µm. The existence of these sources is suggested by the large far-IR and sub-millimeter backgrounds (Puget et al. 1996). These luminous infrared sources are very common at z > 2, over an order of magnitude as common as their local counterparts (Chapman et al. 2003a). With the launch of the Spitzer Space Telescope the study of these galaxies is very much in progress, although important features of these galaxies are already known. Luminous infrared galaxies in the nearby universe are found to be heated by a mixture of AGN activity and star formation (Sanders & Mirabel 1996), although the most luminous sources with L > 1012 L are nearly all starburst-induced major mergers. The relative contributions at high redshift is also a mixture of these two types, although preliminary Spitzer observations show that the spectral energy distributions of high redshift luminous infrared galaxies are largely composed of star forming systems (Frayer et al. 2004).
The rest-frame UV (or observed optical) properties of sub-mm sources observed with the Hubble Space Telescope reveal that these galaxies would be included in magnitude limited optical studies (such as the HDF) and that their structures are very peculiar in appearance (e.g., Chapman et al. 2003b). There are two pieces of evidence that suggests these morphologically peculiar sub-mm/radio selected luminous infrared galaxies at z > 2 are undergoing major galaxy mergers. The first is the very large line widths found in CO mapping (Genzel et al. 2004). Galaxies with similar large velocity widths in the nearby universe are undergoing major mergers (Conselice et al. 2000b). The other evidence is that the morphologies of sub-mm sources are generally peculiar (e.g., Chapman et al. 2003b) and are quantitatively consistent with undergoing major mergers (Conselice et al. 2003b). The merger fraction for sub-mm detected galaxies is in fact higher than for the most massive Lyman-break galaxies (Conselice et al. 2003b). This suggests that the most massive galaxies at high redshift, which are probably these sub-mm sources (Tecza et al. 2004), are forming by mergers.