| We present the results of a multiwavelength study of the role mergers and (U)LIRGs have played in galaxy evolution using the COSMOS data set. First, we investigate the evolution of the galaxy pair fraction out to z = 1.2 by selecting pairs where both members are > L⋆V and have projected separations < 20 kpc. We find that the pair fraction can be fit as a power law of the form (1 + z) n, where n = 3.1 +/- 0.1. We then present a complete sample of 1505 70 mum selected galaxies over the redshift range 0.01 < z < 3.5. The 70 mum data point allows us to measure a total infrared luminosity, LIR, to a greater accuracy than has been possible before at these redshifts with detections at 24 mum only. The multiwavelength coverage has allowed us to compile detailed SEDs from the ultraviolet to the far-infrared. We have identified 355 sources with signatures of an AGN using five selection methods and find that the fraction of galaxies with an AGN increases systematically with L IR. A morphological analysis of these objects finds that the fraction of major mergers also increases with LIR and that nearly 70% of the most luminous sources are major mergers. Likewise, the fraction of spiral galaxies drops dramatically with LIR. The rest-frame U -- V colors of the objects in our sample peak in the green valley in between the blue cloud and the red sequence. We argue that given the number of gas-rich mergers we observe in our sample, and the relatively short timescale in which a merger would be in the (U)LIRG phase, there are enough mergers taking place to account for the formation of massive ellipticals on the red sequence. Finally, we analyze the spectroscopic properties of 380 of our sources in the redshift range 0 < z < 1 and find that at low redshift, the standard diagnostics work well to select AGN, star-forming galaxies, and composite galaxies, however, at high redshift more work is needed to find accurate methods of separating starbursts from AGN. It is clear that a combination of AGN selection using line diagnostics and continuum observations is the most robust way to study the AGN census among (U)LIRGs. |
