Optical imaging of ultraluminous IRAS galaxies: How many are mergers?

We present the results of R-band imaging observations of a sample of 60 ultraluminous IRAS galaxies (ULIRGs). All the objects are taken from our ULIRG survey. The bulk of these objects are in a magnitude-limited subsample of this survey with B-J < 19.5, consisting of 56 objects. We find that 51 of the 56 main-sample objects (91 per cent) are merging and/or disturbed systems, whilst four of the remaining five have close, usually disturbed, companions. Only one of the main sample of objects is isolated and quiescent. The result that almost all of these ULIRGs are disturbed systems contradicts some of the earlier observations. We consider possible origins for this discrepancy, and conclude that deeper images of isolated and quiescent ULIRGs may well reveal previously undetected signs of disturbance.