Evidence for multiple mergers among ultraluminous infrared galaxies: Remnants of compact groups?

In a large sample of ultraluminous infrared galaxies (ULIRGs) imaged with t he Hubble Space Telescope, 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 conservati vely 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 ca ses and discuss the possibility that the progenitors of ULIRGs may be the m ore classical, weakly interacting compact groups of galaxies (see Hickson). An evolutionary progression is consistent with the results: from compact g roups to pairs to ULIRGs to elliptical galaxies. The last step follows the blowout of gas and dust from the ULIRG.