On the nature of high luminosity ULIRGs

We present the preliminary results for a sample of 21 high luminosity ULIRGs (LIR > 1012.3 L⊙), selected from the 1Jy sample. Based on R band imaging we have performed an analysis of their morphologies. The main result obtained is that about half of the host galaxies are represented by spheroids and in the other half an exponential disk has been detected or they show still trace of a merging process going on. Regarding the relation between morphology and nuclear activity it seems that more advanced mergers appear in the most active galaxies. Hence the data support the evolutionary scheme proposed by Sanders 15 years ago that ULIRGs can be the earliest precursors of optical QSOs.