Abstract
Since the discovery of ultra-luminous infrared galaxies (hereafter ULIRGs), the origin of their huge luminosities has been a controversial issue (see, e.g., Sanders & Mirabel 1996). The peculiar morphologies of ULIRGs indicate that they are interacting/merging systems. Optical and near-infrared spectroscopic observations show that a starburst is present in almost all ULIRGs. These lines of evidence suggest that ULIRGs are starbursts, whose activity was induced by an interaction between galaxies. On the other hand, optical spectroscopic observations also show that a significant fraction of galaxies with “warm” infrared colors (i.e., high f25/f60 ratios) are classified as Seyferts. Spectropolarimetry and near-infrared spectroscopy also show that many “warm” galaxies harbor hidden AGNs. On the basis of these results, Sanders et al. (1988) proposed an evolutionary scenario from ULIRGs to optical quasars. They identified “cold” (small f25/f60 ratio) ULIRGs as those at an initial, dust-enshrouded stage which are energetically dominated by starbursts, and “warm” ULIRGs as
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