HST/WFPC2 imaging of the QDOT ultraluminous infrared galaxy sample

We present HST WFPC2 V-band imaging for 23 ultraluminous infrared galaxies (ULIRGs) taken from the QDOT redshift survey. The fraction of sources obser ved to be interacting is 87 per cent. Most of the merging systems show a nu mber of compact 'knots', whose colour and brightness differ substantially f rom their immediate surroundings. Colour maps for nine of the objects show a non-uniform colour structure. Features include blue regions located towar ds the centres of merging systems which are Likely to be areas of enhanced star formation, and compact red regions which are likely to be dust shroude d starbursts or active Galactic nuclei. The host galaxies of the quasi-stel lar objects (QSOs) in the sample were found to be either interacting system s or ellipticals. Our data show no evidence that ULIRGs are a simple transi tion stage between galaxy mergers and QSOs. We propose an alternative model for ULIRGs based on the morphologies in our sample and previous N-body sim ulations. Under this model ULIRGs as a class are much more diverse than a s imple transition between galaxy merger and QSO. The evolution of IR power s ource and mer er morphology in ULIRGs is driven solely by the local environ ment and the morphologies of the merger progenitors.