Evidence for Multiple Mergers among Ultraluminous IR Galaxies (ULIRGs): Remnants of Compact Groups?

In a large sample of ULIRGs imaged with HST, we have identified a significant subsample that shows evidence for multiple mergers. The evidence is seen among two classes of ULIRGs: (1) those with multiple remnant nuclei in their core, sometimes accompanied by a complex system of tidal tails; and (2) those that are in fact dense groupings of interacting (soon-to-merge) galaxies. We conservatively estimate that, in the redshift range 0.05<z<0.20, at least 20 (out of 99) ULIRGs satisfy one or both of these criteria. We present several cases and discuss the possibility that the progenitors of ULIRGs may be the more classical weakly interacting compact groups of galaxies (Hickson 1997). An evolutionary progression is consistent with the results: from compact groups to pairs to ULIRGs to ellipticals. The last step follows the blowout of gas and dust from the ULIRG.