Galaxy mergers can transform the type of the parent galaxy into another and, dur- ing their occurrence, drive phenomena that are extraordinary compared to the pro- cesses that take place in quiescent galaxies. Specifically, simulations and observa- tions show that they trigger star formation events, and young objects at least as massive as globular clusters can be formed. Among the merger environments, lumi- nous (LIRGs; Lbol ∼ LIR = L[8−1000μm] = 1011-1012 L⊙ ) and ultraluminous (ULIRGs; LIR = L[8−1000μm] = 1012-1013 L⊙) infrared galaxies show the most extreme cases of star formation.

It is not surprising that the studies carried out so far on (U)LIRGs have found young compact star forming regions. However, only very few studies have been carried out so far for these kind of systems. This thesis work is devoted to the analysis of compact star forming regions (knots) in a representative sample of 32 (U)LIRGs, the largest sample used for this kind of study in these systems. The project is based mainly on optical high angular resolution images taken with the ACS and WFPC2 cameras on board the HST telescope, data from a high spatial resolution simulation of a major galaxy encounter, and with the combination of optical integral field spectroscopy (IFS) taken with the INTEGRAL (WHT) and VIMOS (VLT) instruments. This is the first time that such combination of different types (photometric, spectroscopic and numerical) of a large amount of data, and such detailed study is performed on these systems. A few thousand knots –a factor of more than one order of magnitude higher than in previous studies– are identified and their photometric properties are characterized as a function of the infrared luminosity of the system and on the interaction phase. These properties are compared with those of compact objects identified in simulations of galaxy encounters. Finally, and with the additional use of IFS data, we search for suitable candidates to tidal dwarf galaxies, setting up constraints on the formation of these objects for the (U)LIRG class. The main findings and conclusions are summarized as follows: