Abstract

We have carried out an optical and infrared study of 24 ultraluminous infrared galaxies in the southern hemisphere (SULIRGs). This flux limited, complete sample, has been extracted from the redshift survey of IRAS galaxies compiled by \cite[Strauss et al. (1992).]{Strauss92} It includes systems with a 60 micron IRAS flux greater than 3 Jy and a far-infrared luminosity greater than 6.5 1011 Lo (Hz). With the ESO New Technology Telescope, we have obtained high resolution optical images in the R band of all SULIRGs, and with the ESO/MPI 2.2-meter telescope, near infrared J, H and K images for most of them. Low and high resolution spectra have been taken with the 4-meter telescope of the Cerro Tololo Inter-American Observatory. The nature and morphology of all objects are consistent with the wide-spread idea that most, if not all, galaxies with an extreme far-infrared luminosity are close interacting or merging systems. The galaxies that appear as isolated and devoid of tidal features exhibit several isophotal distortions in their main body. However three cases are more ambiguous. They are indeed interacting systems, but their companions are at a distance greater than 40 kpc. We find in the SULIRG sample a significant proportion of objects having AGN like spectra: 55 +/- 6 %, including one Seyfert 1 galaxy. Partly based on observations collected at the European Southern Observatory, La Silla, Chile.