Optical structure of a large sample of ultraluminous IRAS galaxies

We present deep images for 34 out of a new sample of 36 ultraluminous Infrared Astronomical Satellite (IRAS) galaxies (ULGs), as well as seven additional high-luminosity objects, selected from the North Galactic Wedge selected area and QDOT all-sky redshift surveys. Previous charge coupled device (CCD) data and Sky Survey material are used in the examination of the two remaining ULGs. All but one of the 43 high-luminosity and ultraluminous IRAS galaxies are classified according to the seven-band system of Lawrence et al., with the exception being unclassifiable due to the proximity of a bright star. Of the total usable sample of 42, we find that 28/42 (67 per cent) are interacting or merging, 4/42 (10 per cent) are in a close pairing but show no obvious signs of an interaction, and 10/42 (23 per cent) are isolated or have only distant companions. Given the clear preponderance of merging systems in infrared-luminous objects, the existence of a substantial fraction of exceptions is interesting. The starburst trigger in these cases is unknown. We discuss spectroscopic data on the galaxies and present new spectra of two objects.