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 Strauss et al. (1992). It includes
systems with a 60 m IRAS flux greater than
3 Jy and a far-infrared luminosity greater than
6:5 1011 L (H0 = 75 km s−1 Mpc−1). 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 nd
in the SULIRG sample a signi cant proportion of objects
having AGN like spectra: 55 6%, including one Seyfert
1 galaxy.