A Multiple Merger Model for the Origin of Ultraluminous Infrared   Galaxies
  It has been often considered that the dissipative collapse caused by a merger of two gas-rich galaxies is responsible for the intense nuclear starbursts or the nonthermal quasar activity in ultraluminous infrared galaxies due to the efficient fueling induced by it. It is also widely known that Ultraluminous Infrared Galaxies (ULIGs) are often found in merging systems. Some ULIGs, such as Arp 220, show two compact starburst regions which are considered to be associated with two galactic nuclei in the process of merging. However, since a merger between two galaxies may make only one compact starburst region, we suggest the possibility that double-nucleus ULIGs are composed of two merging nuclei, each of which contains a couple of galactic nuclei. 
Yoshiaki Taniguchi and Yasuhiro Shioya